QmlDesigner: Use local timeline

We should not use m_graphicsScene->currentTimeline() from the widget.

Change-Id: If31e57884622722818bc1b3254bda98434e65c1e
Reviewed-by: Knud Dollereder <knud.dollereder@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

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

@@ -417,7 +417,7 @@ void TimelineWidget::init()
         m_statusBar->clear();
     }
 
-    invalidateTimelineDuration(m_graphicsScene->currentTimeline());
+    invalidateTimelineDuration(currentTimeline);
 
     m_graphicsScene->setWidth(m_graphicsView->viewport()->width());