Keep unique_identifier of PDF service items.

openlp/plugins/presentations/lib/messagelistener.py

@@ -342,6 +342,7 @@ class MessageListener(object):
             item.iconic_representation = item_cpy.icon
             item.main = item_cpy.main
             item.theme = item_cpy.theme
+            item.unique_identifier = item_cpy.unique_identifier
             # When presenting PDF/XPS/OXPS, we are using the image presentation code,
             # so handler & processor is set to None, and we skip adding the handler.
             self.handler = None

tests/openlp_plugins/presentations/test_messagelistener.py

@@ -98,6 +98,7 @@ def test_start_pdf_presentation(media_mock, media_item):
     mock_item = MagicMock()
     mock_item.processor = 'Pdf'
     mock_item.get_frame_path.return_value = "test.pdf"
+    expected_identifier = mock_item.unique_identifier
     media_item.generate_slide_data = MagicMock()
     ml = MessageListener(media_item)
     ml.media_item = media_item
@@ -109,6 +110,9 @@ def test_start_pdf_presentation(media_mock, media_item):
     # THEN: The handler should be set to None
     assert ml.handler is None, 'The handler should be None'
 
+    # THEN: unique_identifier should still be the same.
+    assert mock_item.unique_identifier == expected_identifier, 'unique_identifier should not be changed'
+
 
 def test_add_handler_failure():
     """