[pitivi] elements: Do not try to call methods on None pointer

[Thibault Saunier <tsaunier src gnome org>]

commit 34969db3af70e6b3cfc900cbfd125a76c43cb24e
Author: Thibault Saunier <tsaunier gnome org>
Date:   Tue Aug 9 20:48:09 2016 -0400

    elements: Do not try to call methods on None pointer
    
    The TrackElement might be outside the its parent while moving between
    layers.

 pitivi/timeline/elements.py |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/timeline/elements.py b/pitivi/timeline/elements.py
index 4732d01..3758eba 100644
--- a/pitivi/timeline/elements.py
+++ b/pitivi/timeline/elements.py
@@ -700,7 +700,7 @@ class VideoSource(TimelineElement):
         asset_width = sinfo.get_width()
         asset_height = sinfo.get_height()
         parent = video_source.get_parent()
-        if not self.__videoflip:
+        if parent and not self.__videoflip:
             for track_element in parent.find_track_elements(
                     None, GES.TrackType.VIDEO, GES.BaseEffect):