redundant remove call with HandlerManager

I'm getting an exception trying to remove a HandlerManager. Although
it may not be set, I don't understand this exception because I'm
trapping the exception, even testing for a null HandlerManager.
What's wrong? Can't this sort of error be trapped.

The error is in this method:

public static void cancelWindowClosingLogoff() {
try {
if (windowClosingHandler != null)
windowClosingHandler.removeHandler();
}
catch (Exception e) {
; // probably never been set
}
}

The stack trace, up to windowClosingHandler.removeHandler() above is:

Uncaught exception: java.lang.AssertionError: redundant remove call
at
com.google.web.bindery.event.shared.SimpleEventBus.doRemoveNow(SimpleEventBus.java:
217)
at
com.google.web.bindery.event.shared.SimpleEventBus.doRemove(SimpleEventBus.java:
107)
at com.google.gwt.event.shared.HandlerManager
$Bus.doRemove(HandlerManager.java:42)
at com.google.web.bindery.event.shared.SimpleEventBus
$1.removeHandler(SimpleEventBus.java:163)
at
com.google.gwt.event.shared.LegacyHandlerWrapper.removeHandler(LegacyHandlerWrapper.java:
26)
...

I'm running GWT 2.3.

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