[pitivi] timeline: Exclude transitions when grouping

[Alexandru Băluț <alexbalut src gnome org>]

commit 2217a49d062d07b58f652b1a253921b784e192e6
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Mon Jun 13 23:02:27 2022 +0200

    timeline: Exclude transitions when grouping
    
    Fixes #2500

 pitivi/utils/timeline.py | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)
---
diff --git a/pitivi/utils/timeline.py b/pitivi/utils/timeline.py
index 4eb91d000..89f228416 100644
--- a/pitivi/utils/timeline.py
+++ b/pitivi/utils/timeline.py
@@ -217,11 +217,16 @@ class Selection(GObject.Object, Loggable):
             have only non-serializable ancestors.
         """
         toplevels = set()
-        for obj in self._clips:
-            if not obj.timeline:
-                # The element has been removed from the timeline. Ignore it.
+        for ges_clip in self._clips:
+            if not ges_clip.timeline:
+                # The clip has been removed from the timeline. Ignore it.
                 continue
-            toplevel = obj.get_toplevel_parent()
+
+            if isinstance(ges_clip, GES.Transition):
+                # The transitions are updated automatically, no need to group them etc.
+                continue
+
+            toplevel = ges_clip.get_toplevel_parent()
             toplevels.add(toplevel)
 
         group = GES.Group.new()