BUG: Fix qMRMLTreeView memory leak

vtkCollectionIterator and vtkCollection had still a reference count of 1
at exit.
Regression from r21523

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21588 3bd1e089-480b-0410-8dfb-8563597acbee

Libs/MRML/Widgets/qMRMLTreeView.cxx

@@ -863,6 +863,7 @@ void qMRMLTreeView::loadTreeExpandState()
     }
   // Clear the vtkCollection now
   d->ExpandedNodes->RemoveAllItems();
+  iter->Delete();
 }
 
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