Right click on workspace after deleting a node throws "node can't be null"

[André Panisson]

Way to reproduce:
* Launch Gephi
* Start a new project
* Add a node using the node pencil
* Using the Rectangle Selection, select the node, Right click, Delete and yes to proceed
* Right click on the empty workspace

java.lang.IllegalArgumentException: node can't be null
 at org.gephi.graph.dhns.graph.AbstractGraphImpl.checkNode(AbstractGraphImpl.java:120)
 at org.gephi.graph.dhns.graph.HierarchicalGraphImpl.getDescendantCount(HierarchicalGraphImpl.java:271)
 at org.gephi.visualization.bridge.DHNSEventBridge.canExpand(DHNSEventBridge.java:80)
 at org.gephi.visualization.apiimpl.GraphContextMenu.getMenu(GraphContextMenu.java:77)
 at org.gephi.visualization.swing.StandardGraphIO.mouseClicked(StandardGraphIO.java:193)
 at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:253)
 at java.awt.Component.processMouseEvent(Component.java:6266)
 at java.awt.Component.processEvent(Component.java:6028)
 at java.awt.Component.dispatchEventImpl(Component.java:4630)
 at java.awt.Component.dispatchEvent(Component.java:4460)
[catch] at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
 at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:137)
 at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
 at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
 at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
 at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Seems that the error appears only if right-clicking the workspace immediately after deleting the node.