[pitivi] previewers: Reset _thumb_cb_id when removing the GSource inside its callback

[Thibault Saunier <tsaunier src gnome org>]

commit 5f72f13356617b64fe74af5d4f136228e7ac6ac3
Author: Thibault Saunier <tsaunier gnome org>
Date:   Sun Aug 27 08:05:32 2017 -0300

    previewers: Reset _thumb_cb_id when removing the GSource inside its callback
    
    Avoiding warnings when stopping the generation after the callback has
    been called.

 pitivi/timeline/previewers.py |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index eec5e7c..02d5041 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -559,6 +559,7 @@ class VideoPreviewer(Previewer, Zoomable, Loggable):
                            Gst.SeekType.NONE, -1)
 
         # Remove the GSource
+        self._thumb_cb_id = None
         return False
 
     def _autosave(self):