The hook in
TextMorph>>#keystroke:  to search for keys in the keymapper  looks like 
:

keyStroke: evt
	"Handle a keystroke event."
	| action |
	
	"hook for keymapper"
	[(self keyMapper match: evt) ifTrue:[^self].]
		on: Error
		do: [ :ex |  ex signal ].
	
	"rest of method ... "

This relaunches the exception, but the debuggers' logic for creating 
the additional
button on MNU is wrong.

So replacing it with :

keyStroke: evt
	"Handle a keystroke event."
	| action |
	
	"hook for keymapper"
	(self keyMapper match: evt) ifTrue:[^self].

i.e. not catching the exception is a workaround