"Attempt to insert nil value" error message

[David Balcom]

Greetings,

I just finished a nice wireframe in OmniGraffle 2.08. Only now I can't 
reopen the document, and I get this error message in a dialog box:

*** -[NSPlaceholderDictionaryinitWithObjects:forKeys:count]: attempt to 
insert nil value

Any ideas how I can open this and continue with my work? I'd love to 
continue using OmniGraffle, but I'm not going to use it if I can't count on 
opening my own documents ...

Best,
dbalcom at mindspring.com