[pitivi] proxy: Check for proxies only on UriClips

[Thibault Saunier <tsaunier src gnome org>]

commit 73c89884da59cf2a9572316d03032987b37839d8
Author: Thibault Saunier <tsaunier igalia com>
Date:   Mon Mar 30 17:54:16 2020 -0300

    proxy: Check for proxies only on UriClips

 pitivi/utils/proxy.py | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/pitivi/utils/proxy.py b/pitivi/utils/proxy.py
index 87ba79a6..40fba6ba 100644
--- a/pitivi/utils/proxy.py
+++ b/pitivi/utils/proxy.py
@@ -475,6 +475,8 @@ class ProxyManager(GObject.Object, Loggable):
             target_uri = self.get_target_uri(asset)
 
             for clip in self.app.project_manager.current_project.ges_timeline.iter_clips():
+                if not isinstance(clip, GES.UriClip):
+                    continue
                 if self.get_target_uri(clip.props.uri) == target_uri:
                     if clip.props.in_point + clip.props.duration > duration:
                         new_duration = duration - clip.props.in_point