QmlDesigner: Fix crash

There is no guarantee that the timeline is valid at this point.

Change-Id: I48a31b1f4c305c4764d8bebee9de16e51aec4a47
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/timelineeditor/timelinegraphicsscene.cpp

@@ -174,11 +174,13 @@ void TimelineGraphicsScene::invalidateLayout()
 
 void TimelineGraphicsScene::updateKeyframePositionsCache()
 {
+    if (currentTimeline().isValid()) {
         auto kfPos = keyframePositions();
         std::sort(kfPos.begin(), kfPos.end());
         kfPos.erase(std::unique(kfPos.begin(), kfPos.end()), kfPos.end()); // remove duplicates
 
         m_keyframePositionsCache = kfPos;
+    }
 }
 
 // snap a frame to nearest keyframe or ruler tick