Chris Muller uploaded a new version of ToolBuilder-Morphic to project The Trunk:
http://source.squeak.org/trunk/ToolBuilder-Morphic-cmm.154.mcz

==================== Summary ====================

Name: ToolBuilder-Morphic-cmm.154
Author: cmm
Time: 22 December 2015, 3:33:18.425 pm
UUID: 9c0f9cf0-1b21-448f-8401-f69edc04e17e
Ancestors: ToolBuilder-Morphic-mt.153

Encountered the model already nil'd out before entering PluggableSystemWindow>>#delete on occasion.  Protect against that.

=============== Diff against ToolBuilder-Morphic-mt.153 ===============

Item was changed:
  ----- Method: PluggableSystemWindow>>delete (in category 'initialization') -----
  delete
+ 	(model notNil and: [ closeWindowSelector notNil ]) ifTrue: [ model perform: closeWindowSelector ].
+ 	super delete!
- 	closeWindowSelector ifNotNil:[model perform: closeWindowSelector].
- 	super delete.
- !