[pitivi] elements: Fix ALLOWED_ONLY_ONCE_EFFECTS check

[Thibault Saunier <tsaunier src gnome org>]

commit 9863fe98e20c8c0f78c7c09b847399db8d14dfe3
Author: Suhas Nayak <suhas2go gmail com>
Date:   Fri Jul 21 13:09:15 2017 +0000

    elements: Fix ALLOWED_ONLY_ONCE_EFFECTS check
    
    find_track_elements is a method of the GES.Clip.
    
    This fixes T7785
    
    Reviewed-by: Thibault Saunier <tsaunier gnome org>
    Differential Revision: https://phabricator.freedesktop.org/D1796

 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 ffaa9c7..0b2870d 100644
--- a/pitivi/timeline/elements.py
+++ b/pitivi/timeline/elements.py
@@ -941,7 +941,7 @@ class Clip(Gtk.EventBox, Zoomable, Loggable):
         """
         factory_name = effect_info.effect_name
         if factory_name in ALLOWED_ONLY_ONCE_EFFECTS:
-            for effect in self.find_track_elements(None, GES.TrackType.VIDEO,
+            for effect in self.ges_clip.find_track_elements(None, GES.TrackType.VIDEO,
                                                    GES.BaseEffect):
                 for elem in effect.get_nleobject().iterate_recurse():
                     if elem.get_factory().get_name() == factory_name: