ObjectUnderMouseCursor is set to last node after node is removed

[assafwo1]

reproduce steps:

1. create a new gviewer
2. hover over gviewer
3. assert ObjectUnderMouseCursor is null
4. add a new node
5. hover over the new node
6. assert ObjectUnderMouseCursor is the node
7. remove the node
8. assert ObjectUnderMouseCursor is null

issue: step 8 fails.

[assafwo1]

    public IViewerObject ObjectUnderMouseCursor
    {
        get
        {
            // >>>>>>>>>>>>>>>this function can bring a stale object<<<<<<<<<<<<<<<
            var location = Mouse.GetPosition(_graphCanvas);
            if (!(_objectUnderMouseDetectionLocation == location))
                UpdateWithWpfHitObjectUnderMouseOnLocation(location, MyHitTestResultCallbackWithNoCallbacksToTheUser);
            return GetIViewerObjectFromObjectUnderCursor(_objectUnderMouseCursor);
        }
    }

[assafwo1]

work around: gViewer1.ObjectUnderMouseCursor is Microsoft.Msagl.GraphViewerGdi.DNode // unreliable gViewer1.ObjectUnderMouseCursor is Microsoft.Msagl.GraphViewerGdi.DNode && gViewer1.Graph.NodeCount != 0 // reliable