Describe the problem: what do you get? what do you expect?

When you modfiy a block that is not on the stack (like [ self halt ]) and you browse
it, you get: 
DNU: DebugContext>>codeTextMorph

How to reproduce the problem: step by step if necessary
create a method in a class Class:

sendBlock
   ^ [ self halt ]

And call :

Class new sendBlock value.

It will open the debugger.
Now modify the block content (remove the halt for example)
Save, click on browse in the notification window.
You should get the error.

Additional information: platform, context which may impact the problem

W7 - Moose 5.0

Please fill in the labels with the following information:
* Type-Defect
* Component-GTDebugger