Returns the thread in which this event has occurred.
In rare cases, this event may occur in a debugger system thread within the target VM. Debugger threads take precautions to prevent these events, but they cannot be avoided under some conditions, especially for some subclasses of {@link java.lang.Error}. If the event was generated by a debugger system thread, the value returned by this method is null, and if the requested suspend policy for the event was {@link com.sun.jdi.request.EventRequest#SUSPEND_EVENT_THREAD}, all threads will be suspended instead, and the {@link EventSet#suspendPolicy} will reflect this change.
Note that the discussion above does not apply to system threads created by the target VM during its normal (non-debug) operation.
@return a {@link ThreadReference} which mirrors the event's thread inthe target VM, or null in the rare cases described above.