The bug manifests itself as IZ 140788.

To observe what is described below put a breakpoint 
 inside org.openide.nodes.EntrySupport$Default.getArray() method.

While processing  expansion of a node in project tree 
the AWT-EventQueue thread posts a request into RequestProcessor
at org.openide.explorer.view.TreeView.prepareWaitCursor(). Thus 2 threads 
would execute  org.openide.nodes.EntrySupport$Default.getArray() method.


Scenario 1.
If AWT-EventQueue thread gets first into the getArray() method it 
will successfully execute 

            if (doInitialize) {
...
                    children.callAddNotify();
...

"children calculation" branch, while RequestProcessor will be waiting 
in the "else" branch for the calculation to be completed.


Scenario 2.
If RequestProcessor thread gets first into the getArray() method it 
will be executing the "children calculation" branch. Unfortunately 
the AWT-EventQueue (which at the moment is processing the node 
expansion mouse click) willn't be waiting on the "else" branch for
 the "children calculation" to be completed. The reason is that 
the AWT-EventQueue has already obtained these Children's MUTEX.readAccess() 
back at
         at org.openide.explorer.view.TreeView$ExplorerTree$GuardedActions.<init>
(TreeView.java:2043)

Thus the check in the "else" branch of getArray() method 

                    if (Children.MUTEX.isReadAccess() || Children.MUTEX.isWriteAccess() || (initThread ==
Thread.currentThread())) {                

will produce "true". So the getNodes() executed in AWT-EventQueue at


        ...
        at org.openide.nodes.EntrySupport$Default.getNodes(EntrySupport.java:153
)
        ...
        at org.openide.explorer.view.VisualizerNode.getChildren(VisualizerNode.j
ava:250)
        at org.openide.explorer.view.VisualizerNode.getChildCount(VisualizerNode
.java:324)
        at javax.swing.tree.DefaultTreeModel.getChildCount(DefaultTreeModel.java
:168)
        ...

would return empty set of children.   

The RequestProcessor who is executing 
                    children.callAddNotify();

in the "children calculation" branch wouldn't be able to execute it as 
the writeRequests generated for the mutex are put on hold until 
readAccess is relinquished by AWT-EventQueue ( ie. until AWT-EventQueue 
returns well above (in the call stack) getNodes() whcih already 
produced empty set of children)

Thus mouse click to expand a node would produce no children-nodes 
under the node.