[pitivi] previewers: Fix stacktrace when removing a clip

[Mathieu Duponchelle <mathieudu src gnome org>]

commit 3fd9b9a2fe6b6f2a4d6251c2c2af487a2e2f071e
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Mon Jan 6 09:27:38 2014 +0100

    previewers: Fix stacktrace when removing a clip
    
    Queued clip previewers which are waiting are removed from the queue and
    everything is fine, but if the PreviewGeneratorManager starts to work on
    a previewer and its clip is removed from the timeline, the previewer will
    unset its pipeline attribute and an exception occurs.

 pitivi/timeline/previewers.py |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index f2bdb74..8876026 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -263,6 +263,11 @@ class VideoPreviewer(Clutter.ScrollActor, PreviewGenerator, Zoomable, Loggable):
         GLib.idle_add(self._startThumbnailing, priority=GLib.PRIORITY_LOW)
 
     def _startThumbnailing(self):
+        if not self.pipeline:
+            # Can happen if stopGeneration is called because the clip has been
+            # removed from the timeline after the PreviewGeneratorManager
+            # started this job.
+            return
         self.debug('Now generating thumbnails for: %s', filename_from_uri(self.uri))
         query_success, duration = self.pipeline.query_duration(Gst.Format.TIME)
         if not query_success or duration == -1: