I'm not sure what caused it, but I just hit an emergency evaluator on
updating a clean 4.5 (as in the Squeak4.5.image in the build scripts).

It goes something like this:

***System error handling failed***
Original error: MessageNotUnderstood: UndefinedObject>>at:ifPresent:.
    Debugger error: MessageNotUnderstood: UndefinedObject>>at:ifPresent::
Debugger class(Object)>>primitiveError:
[] in [] in Debugger class>>morphicOpenOn:context:label:contents:fullView:
BlockContext>>cull:
[] in MethodContext(ContextPart)>>handleSignal:
BlockClosure>>ensure:
MethodContext(ContextPart)>>handleSignal:
MessageNotUnderstood(Exception)>>signal
UndefinedObject(Object)>>doesNotUnderstand: #at:ifPresent:
Environment>>at:ifPresentAndInMemory:
SmalltalkImage>>at:ifPresentAndInMemory:
Debugger>>process:controller:context:
[] in [] in Debugger class>>morphicOpenOn:context:label:contents:fullView:

and so on. I'm strongly tempted to point at Environments-cwp.27
because of that call to #at:ifPresentAndInMemory: which uses
Environment's contents ivar prior to this change and the new
declarations ivar after. A missing migration step over the shape
change?

frank