null object RTE when closing the parent of an opened empty child node in tree

I have a tree with an XMLListCollection as data provider. I have used isBranch="true" in the underlying XML to indicate empty nodes that should display as folders. When I click open an empty child node in the tree and then close the parent node without first closing the empty child, I get a null object RTE. If I close the empty child node first, then I don't get the error. From the error message (below) it seems Flash Player is maybe looking for nonexistent children of the empty node (?) TypeError: Error #1009: Cannot access a property or method of a null object reference.       at mx.controls::Tree/getOpenChildrenStack()[E:\dev\flex_201_borneo\sdk\frameworks\mx\controls\Tree.as:2404]       at mx.controls::Tree/getOpenChildrenStack()[E:\dev\flex_201_borneo\sdk\frameworks\mx\controls\Tree.as:2411]       at mx.controls::Tree/buildUpCollectionEvents()[E:\dev\flex_201_borneo\sdk\frameworks\mx\controls\Tree.as:2192]       at mx.controls::Tree/http://www.adobe.com/2006/flex/mx/internal::onTweenEnd()[E:\dev\flex_201_borneo\sdk\frameworks\mx\controls\Tree.as:2017]       at mx.effects::Tween/endTween()[C:\dev\flex_201_gmc\sdk\frameworks\mx\effects\Tween.as:510]       at mx.effects::Tween/http://www.adobe.com/2006/flex/mx/internal::doInterval()[C:\dev\flex_201_gmc\sdk\frameworks\mx\effects\Tween.as:548]       at mx.effects::Tween$/timerHandler()[C:\dev\flex_201_gmc\sdk\frameworks\mx\effects\Tween.as:177]       at flash.utils::Timer/_timerDispatch()       at flash.utils::Timer/tick() Debug mode highlights line 2404 of the Tree class:             var cursor:IViewCursor = ICollectionView(iterator.view).createCursor();