All of a sudden I can't export or publish

I’m working on a test site and have ben publishing small updates many many times a day
After a minor change
All of a sudden I cant export or publish and get this error message.
Ive tried closing RW and restarting Mac. No idea what the message means!
Any help greatly appreciated

Exception while exporting site

*** -[NSURL URLByAppendingPathComponent:]:
component, components, or pathExtension cannot be nil.

Ive fixed it. It was a badly labeled image in Resources