GrailsExceptionResolver null pointer exception

This is the message I got after I hit a button in the gsp with an action="waiver".

23.11.10 09:36:35.054] ERROR errors.GrailsExceptionResolver - null
java.lang.NullPointerException
at java.lang.Thread.run(Thread.java:619)

I had a hard time to figure out this problem. The associated controller had an action:

def waiver = {
}

and the waver.gsp exists.

Now why did the exception happen? The reason was, because the controller has a method called getWaiver().