On a second thought i think there is still something wrong :)

An ExternalStructure class having method #doneCompiling on a class side.
And it get called by ClassBuilder before ExternalStructure being
properly initialized.

Here is a top of call stack:

UndefinedObject(Object)>>doesNotUnderstand: #at:ifAbsent:
[] in ExternalType class>>noticeModificationOf: {[:cls |  type :=
StructTypes     at: cls name     ifAbsent: [].  type   ifNo...]}
ExternalStructure class(Behavior)>>withAllSubclassesDo:
ExternalType class>>noticeModificationOf:
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
ExternalStructure class>>doneCompiling
ClassBuilder>>name:inEnvironment:subclassOf:type:instanceVariableNames:classVariableNames:poolDictionaries:category:unsafe:

-- 
Best regards,
Igor Stasenko AKA sig.