I seem to have the same problem; after clicking on my project-file Rapidweaver (7) responds:
"The (null) theme that this document uses is missing. If you open this project, RapidWeaver will apply the Offroad theme. Are you sure you still want to open this project?"
I wanted to, so I clicked on “Apply Theme and open” but nothing happened. The spooky thing is that all themes, including the used “CADiamond” and Offroad are on my Mac and when I start a new website I can use these and other themes and stacks!
This problem started today after I had set up my new iMac a few weeks ago because my old iMac had crashed. Today I tried to fire up Rapidweaver (7) to edit this website. (Everything had been restored with Timemachine).
The silly thing is that I can build new websites with all themes and stacks, and CADiamond and Offroad work perfectly. Rapidweaver does find and recognise all themes and stacks but only some of my websites refuse to open, not even with the offroad-theme or whatever. Other ones with the same age, are fine…
I really hope anyone can help me out, thank you very much in advantage!
[edit Peter] I just put back an older copy of the two project-files that would not open… PROBLEM SOLVED so the problem must have been damaged files.