Please have a look at http://netbeans.org/bugzilla/show_bug.cgi?id=225851
and http://statistics.netbeans.org/exceptions/detail.do?id=198377

Following exception was unexpectedly thrown to our customers:

java.lang.NullPointerException
        at sun.rmi.transport.DGCClient$EndpointEntry.removeRefEntry(DGCClient.java:333)
        at sun.rmi.transport.DGCClient$EndpointEntry.processPhantomRefs(DGCClient.java:615)
        at sun.rmi.transport.DGCClient$EndpointEntry.access$1400(DGCClient.java:171)
        at sun.rmi.transport.DGCClient$EndpointEntry$RenewCleanThread.run(DGCClient.java:556)
        at java.lang.Thread.run(Thread.java:722)