Preferences: Check for pipeline before removing elements

Fixes #10430

gajim/gtk/video_preview.py

@@ -100,10 +100,12 @@ def _disable_preview(self):
             self._av_pipeline.set_state(Gst.State.NULL)
             if self._av_src is not None:
                 self._av_pipeline.remove(self._av_src)
-            self._av_src = None
             if self._av_sink is not None:
                 self._av_pipeline.remove(self._av_sink)
+
+        self._av_src = None
         self._av_sink = None
+
         if self._av_widget is not None:
             self._ui.video_preview_box.remove(self._av_widget)
             self._ui.video_preview_placeholder.set_visible(True)