The exception is strange also for us.
The reported line (ExtConnectionListener.java:129) contains a method call on a class that must have been supplied by the JDK since java 5.
However, other invocations of that method must have been performed before the specified one.
I can only expect a JVM bug, perhaps a race condition as the class is being replaced by some bytecode optimization process.
Did you see the exception always with the same stack trace?