diff --git a/openlp/core/lib/serviceitem.py b/openlp/core/lib/serviceitem.py
index d7cce51d6..687df9422 100644
--- a/openlp/core/lib/serviceitem.py
+++ b/openlp/core/lib/serviceitem.py
@@ -25,7 +25,6 @@ import datetime
 import logging
 import ntpath
 import os
-import urllib.request
 import uuid
 from copy import deepcopy
 from pathlib import Path
@@ -39,9 +38,9 @@ from openlp.core.common.enum import ServiceItemType
 from openlp.core.common.i18n import translate
 from openlp.core.common.mixins import RegistryProperties
 from openlp.core.common.registry import Registry
+from openlp.core.common.utils import wait_for
 from openlp.core.display.render import remove_tags, render_tags, render_chords_for_printing
-from openlp.core.lib import ItemCapabilities
-from openlp.core.lib import create_thumb
+from openlp.core.lib import create_thumb, image_to_data_uri, ItemCapabilities
 from openlp.core.lib.theme import BackgroundType, TransitionSpeed
 from openlp.core.state import State
 from openlp.core.ui.icons import UiIcons
@@ -70,6 +69,7 @@ class ServiceItem(RegistryProperties):
         self._rendered_slides = None
         self._display_slides = None
         self._print_slides = None
+        self._creating_slides = False
         self.title = ''
         self.slides = []
         self.processor = None
@@ -202,6 +202,8 @@ class ServiceItem(RegistryProperties):
         """
         Create frames for rendering and display
         """
+        wait_for(lambda: not self._creating_slides)
+        self._creating_slides = True
         self._rendered_slides = []
         self._display_slides = []
 
@@ -234,12 +236,14 @@ class ServiceItem(RegistryProperties):
                 }
                 self._display_slides.append(display_slide)
                 index += 1
+        self._creating_slides = False
 
     @property
     def rendered_slides(self):
         """
         Render the frames and return them
         """
+        wait_for(lambda: not self._creating_slides)
         if not self._rendered_slides:
             self._create_slides()
         return self._rendered_slides
@@ -249,6 +253,7 @@ class ServiceItem(RegistryProperties):
         """
         Render the frames and return them
         """
+        wait_for(lambda: not self._creating_slides)
         if not self._display_slides:
             self._create_slides()
         return self._display_slides
@@ -868,6 +873,7 @@ class ServiceItem(RegistryProperties):
     def to_dict(self):
         """
         Convert the service item into a dictionary
+        Images and thumbnails are put in dict as data uri strings.
         """
         data_dict = {
             'title': self.title,
@@ -902,7 +908,7 @@ class ServiceItem(RegistryProperties):
                 full_thumbnail_path = AppLocation.get_data_path() / thumbnail_path
                 if not full_thumbnail_path.exists():
                     create_thumb(Path(self.get_frame_path(index)), full_thumbnail_path, False)
-                item['img'] = urllib.request.pathname2url(os.path.sep + str(thumbnail_path))
+                item['img'] = image_to_data_uri(full_thumbnail_path)
                 item['text'] = str(frame['title'])
                 item['html'] = str(frame['title'])
             else:
@@ -921,7 +927,7 @@ class ServiceItem(RegistryProperties):
                         log.warning('Service item "{title}" is missing a thumbnail or has an invalid thumbnail path'
                                     .format(title=self.title))
                     else:
-                        item['img'] = urllib.request.pathname2url(os.path.sep + str(relative_file))
+                        item['img'] = image_to_data_uri(AppLocation.get_data_path() / relative_file)
                 item['text'] = str(frame['title'])
                 item['html'] = str(frame['title'])
             data_dict['slides'].append(item)
diff --git a/openlp/core/ui/thememanager.py b/openlp/core/ui/thememanager.py
index eb0a956c8..bc33b3f30 100644
--- a/openlp/core/ui/thememanager.py
+++ b/openlp/core/ui/thememanager.py
@@ -738,6 +738,8 @@ class ThemeManager(QtWidgets.QWidget, RegistryBase, Ui_ThemeManager, LogMixin, R
         for theme_name in theme_name_list:
             theme_data = self._get_theme_data(theme_name)
             preview_pixmap = self.progress_form.get_preview(theme_name, theme_data)
+            if preview_pixmap is None:
+                break
             self.save_preview(theme_name, preview_pixmap)
         self.progress_form.close()
         self.load_themes()
diff --git a/openlp/core/ui/themeprogressform.py b/openlp/core/ui/themeprogressform.py
index 86bd5c275..568874feb 100644
--- a/openlp/core/ui/themeprogressform.py
+++ b/openlp/core/ui/themeprogressform.py
@@ -49,11 +49,20 @@ class ThemeProgressForm(QtWidgets.QDialog, UiThemeProgressDialog, RegistryProper
         return super().show()
 
     def get_preview(self, theme_name, theme_data):
+        """
+        Fetch a screenshot of the webengine preview
+
+        :return: Image of the webengine, can be None
+        """
         self.label.setText(theme_name)
         self.progress_bar.setValue(self.progress_bar.value() + 1)
         wait_for(lambda: self.theme_display.is_initialised)
         self.theme_display.set_scale(float(self.theme_display.width()) / self.renderer.width())
-        return self.theme_display.generate_preview(theme_data, generate_screenshot=True)
+        screenshot = self.theme_display.generate_preview(theme_data, generate_screenshot=True)
+        if self.isVisible():
+            return screenshot
+        else:
+            return None
 
     def _get_theme_list(self):
         """Property getter"""
diff --git a/tests/functional/openlp_core/lib/test_serviceitem.py b/tests/functional/openlp_core/lib/test_serviceitem.py
index 14a38d392..7dae51929 100644
--- a/tests/functional/openlp_core/lib/test_serviceitem.py
+++ b/tests/functional/openlp_core/lib/test_serviceitem.py
@@ -24,7 +24,7 @@ Package to test the openlp.core.lib package.
 import os
 import pytest
 from pathlib import Path
-from unittest.mock import Mock, MagicMock, patch, ANY
+from unittest.mock import Mock, MagicMock, patch
 
 from openlp.core.common import ThemeLevel, is_win
 from openlp.core.common.enum import ServiceItemType
@@ -809,7 +809,9 @@ def test_to_dict_text_item(state_media, settings, service_item_env):
     assert result == expected_dict
 
 
-def test_to_dict_image_item(state_media, settings, service_item_env):
+@patch('openlp.core.lib.serviceitem.AppLocation.get_data_path')
+@patch('openlp.core.lib.serviceitem.image_to_data_uri')
+def test_to_dict_image_item(mocked_image_to_data_uri, mocked_get_data_path, state_media, settings, service_item_env):
     """
     Test that the to_dict() method returns the correct data for the service item
     """
@@ -820,6 +822,8 @@ def test_to_dict_image_item(state_media, settings, service_item_env):
     service_item.add_icon = MagicMock()
     FormattingTags.load_tags()
     line = convert_file_service_item(TEST_PATH, 'serviceitem_image_2.osj')
+    mocked_get_data_path.return_value = Path('/path/to/')
+    mocked_image_to_data_uri.side_effect = lambda x: 'your image uri at: {}'.format(x)
 
     with patch('openlp.core.lib.serviceitem.sha256_file_hash') as mocked_sha256_file_hash:
         mocked_sha256_file_hash.return_value = '3a7ccbdb0b5a3db169c4692d7aad0ec8'
@@ -841,7 +845,7 @@ def test_to_dict_image_item(state_media, settings, service_item_env):
         'slides': [
             {
                 'html': 'image_1.jpg',
-                'img': '/images/thumbnails/image_1.jpg',
+                'img': 'your image uri at: /path/to/images/thumbnails/image_1.jpg',
                 'selected': False,
                 'tag': 1,
                 'text': 'image_1.jpg',
@@ -856,7 +860,8 @@ def test_to_dict_image_item(state_media, settings, service_item_env):
 
 
 @patch('openlp.core.lib.serviceitem.AppLocation.get_data_path')
-def test_to_dict_presentation_item(mocked_get_data_path, state_media, settings, service_item_env):
+@patch('openlp.core.lib.serviceitem.image_to_data_uri')
+def test_to_dict_presentation_item(mocked_image_uri, mocked_get_data_path, state_media, settings, service_item_env):
     """
     Test that the to_dict() method returns the correct data for the service item
     """
@@ -870,6 +875,7 @@ def test_to_dict_presentation_item(mocked_get_data_path, state_media, settings,
     image = Path('thumbnails/abcd/slide1.png')
     display_title = 'DisplayTitle'
     notes = 'Note1\nNote2\n'
+    mocked_image_uri.side_effect = lambda x: 'your img uri at: {}'.format(x)
 
     # WHEN: adding presentation to service_item
     with patch('openlp.core.lib.serviceitem.sha256_file_hash') as mocked_sha256_file_hash,\
@@ -898,7 +904,7 @@ def test_to_dict_presentation_item(mocked_get_data_path, state_media, settings,
                 'tag': 1,
                 'text': 'test.pptx',
                 'title': '',
-                'img': ANY
+                'img': 'your img uri at: /path/to/presentations/thumbnails/4a067fed6834ea2bc4b8819f11636365/slide1.png'
             }
         ],
         'theme': None,
diff --git a/tests/openlp_core/ui/test_themeprogressform.py b/tests/openlp_core/ui/test_themeprogressform.py
index 60f715d65..a455c1e43 100644
--- a/tests/openlp_core/ui/test_themeprogressform.py
+++ b/tests/openlp_core/ui/test_themeprogressform.py
@@ -36,7 +36,9 @@ def _get_theme_progress_form():
 
 
 def test_init(qapp):
-    """Test that the ThemeProgressForm is created without problems"""
+    """
+    Test that the ThemeProgressForm is created without problems
+    """
     # GIVEN: ThemeProgressForm class
     # WHEN: An object is instatiated
     # THEN: There is no problem
@@ -46,7 +48,9 @@ def test_init(qapp):
 @patch('openlp.core.ui.themeprogressform.ScreenList')
 @patch('openlp.core.ui.themeprogressform.QtWidgets.QDialog.show')
 def test_show(mocked_show, MockScreenList, settings):
-    """Test that the ThemeProgressForm is created without problems"""
+    """
+    Test that the ThemeProgressForm is created without problems
+    """
     # GIVEN: ThemeProgressForm object
     form = _get_theme_progress_form()
     mocked_screen_list = MagicMock()
@@ -70,7 +74,9 @@ def test_show(mocked_show, MockScreenList, settings):
 @patch('openlp.core.ui.themeprogressform.ScreenList')
 @patch('openlp.core.ui.themeprogressform.QtWidgets.QDialog.show')
 def test_show_divide_by_zero(mocked_show, MockScreenList, settings):
-    """Test that the ThemeProgressForm is created without problems even if there's a divide by zero exception"""
+    """
+    Test that the ThemeProgressForm is created without problems even if there's a divide by zero exception
+    """
     # GIVEN: ThemeProgressForm object
     form = _get_theme_progress_form()
     mocked_screen_list = MagicMock()
@@ -92,7 +98,9 @@ def test_show_divide_by_zero(mocked_show, MockScreenList, settings):
 
 
 def test_get_preview(settings):
-    """Test that the get_preview() method returns a preview image"""
+    """
+    Test that the get_preview() method returns a preview image
+    """
     # GIVEN: ThemeProgressForm object
     Registry.create()
     mocked_renderer = MagicMock()
@@ -100,6 +108,7 @@ def test_get_preview(settings):
     test_theme_name = 'Test Theme'
     test_theme_data = {'name': test_theme_name}
     form = _get_theme_progress_form()
+    form.isVisible = MagicMock(return_value=True)
     form.progress_bar = MagicMock(**{'value.return_value': 0})
     form.label = MagicMock()
     form.theme_display = MagicMock(**{'width.return_value': 192, 'generate_preview.return_value': 'preview'})
@@ -119,6 +128,30 @@ def test_get_preview(settings):
     assert preview == 'preview'
 
 
+def test_get_preview_not_visible(settings):
+    """
+    Test that the get_preview() method does not return a preview image when display is not visible
+    """
+    # GIVEN: ThemeProgressForm object
+    Registry.create()
+    mocked_renderer = MagicMock()
+    Registry().register('renderer', mocked_renderer)
+    test_theme_name = 'Test Theme'
+    test_theme_data = {'name': test_theme_name}
+    form = _get_theme_progress_form()
+    form.isVisible = MagicMock(return_value=False)
+    form.progress_bar = MagicMock(**{'value.return_value': 0})
+    form.label = MagicMock()
+    form.theme_display = MagicMock(**{'width.return_value': 192, 'generate_preview.return_value': 'preview'})
+    form.renderer.width = MagicMock(return_value=1920)
+
+    # WHEN: get_preview() is called
+    preview = form.get_preview(test_theme_name, test_theme_data)
+
+    # THEN: Result should be None
+    assert preview is None
+
+
 def test_theme_list(qapp):
     # GIVEN: ThemeProgressForm object and theme list
     test_theme_list = ['Theme 1', 'Theme 2']