Time out after 10 seconds in wait till loaded fn

also fixes comment, update_preview_images() does not handle the timeout case but in reality it should never happen. (just in the case it does, it won't stall the whole application)

openlp/core/display/render.py

@@ -486,6 +486,24 @@ class ThemePreviewRenderer(LogMixin, DisplayWindow):
             footer_html = 'Dummy footer text'
         return footer_html
 
+    def wait_till_loaded(self):
+        """
+        Wait until web engine page loaded
+        :return boolean: True on success, False on timeout
+        """
+        # Timeout in 10 seconds
+        end_time = time.time() + 10
+        app = Registry().get('application')
+        success = True
+        while not self._is_initialised:
+            if time.time() > end_time:
+                log.error('Timed out waiting for web engine page to load')
+                success = False
+                break
+            time.sleep(0.1)
+            app.process_events()
+        return success
+
     def _wait_and_process(self, delay):
         """
         Wait while allowing things to process

openlp/core/display/window.py

@@ -402,6 +402,8 @@ class DisplayWindow(QtWidgets.QWidget):
         Set the HTML scale
         """
         self.scale = scale
+        # Only scale if initialised (scale run again once initialised)
+        if self._is_initialised:
             self.run_javascript('Display.setScale({scale});'.format(scale=scale * 100))
 
     def alert(self, text, settings):

openlp/core/ui/thememanager.py

@@ -722,6 +722,7 @@ class ThemeManager(QtWidgets.QWidget, RegistryBase, Ui_ThemeManager, LogMixin, R
         theme_name_list = theme_name_list or self.get_theme_names()
         self.progress_form.theme_list = theme_name_list
         self.progress_form.show()
+        self.progress_form.theme_display.wait_till_loaded()
         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)

tests/functional/openlp_core/display/test_window.py

@@ -74,3 +74,33 @@ class TestDisplayWindow(TestCase, TestMixin):
         # THEN: The x11 override flag should not be set
         x11_bit = display_window.windowFlags() & QtCore.Qt.X11BypassWindowManagerHint
         assert x11_bit != QtCore.Qt.X11BypassWindowManagerHint
+
+    def test_set_scale_not_initialised(self, MockSettings, mocked_webengine, mocked_addWidget):
+        """
+        Test that the scale js is not run if the page is not initialised
+        """
+        # GIVEN: A display window not yet initialised
+        display_window = DisplayWindow()
+        display_window._is_initialised = False
+        display_window.run_javascript = MagicMock()
+
+        # WHEN: set scale is run
+        display_window.set_scale(0.5)
+
+        # THEN: javascript should not be run
+        display_window.run_javascript.assert_not_called()
+
+    def test_set_scale_initialised(self, MockSettings, mocked_webengine, mocked_addWidget):
+        """
+        Test that the scale js is not run if the page is not initialised
+        """
+        # GIVEN: A display window not yet initialised
+        display_window = DisplayWindow()
+        display_window._is_initialised = True
+        display_window.run_javascript = MagicMock()
+
+        # WHEN: set scale is run
+        display_window.set_scale(0.5)
+
+        # THEN: javascript should not be run
+        display_window.run_javascript.assert_called_once_with('Display.setScale(50.0);')