[Pythonmac-SIG] [py2app] "An unexpected error has occurred during execution of the main script"

[Michael Glassford]

I get this error running a py2app alias build (one built with option 
alias=True) of an application, with the additional information 
"AttributeError: 'NoneType' object has no attribute 'endswith'; See the 
Console for a detailed traceback." However, no additional information is 
written to the console.

This alias build had been working for a long time. As far as I know, 
there have been no recent configuration changes that should cause this 
problem. If I make a non-alias build, it runs as it should.

As far as I can tell, this error happens before site.py, __boot.py__ are 
executed: if I put print statements at the top of these files, nothing 
is printed. I suppose that's what "during execution of the main script" 
means?

So my question is, how do I debug what's causing this problem?

Mike