Updating protection of timeline_sync.timeline

Author: jonoomph

src/windows/main_window.py

@@ -175,7 +175,7 @@ def closeEvent(self, event):
         self.preview_parent.background.wait(5000)
 
         # Close Timeline
-        if self.timeline_sync:
+        if self.timeline_sync and self.timeline_sync.timeline:
             self.timeline_sync.timeline.Close()
             self.timeline_sync.timeline = None