Hmm yeah it's a bit of an odd case. We store a list of file ID values for the cloud project locally for the recent projects list, so until you try to actually open the file we don't contact Google Drive to see if it's there. Deleted files are exactly the same as normal files in Google Drive, except they have a label saying "trashed = true", so I guess it allows you to open the file if you still have the ID.
Ok, so if the file has a "trashed" tag when you try to open it
then dont let C3 open it it and instead display a message
"sorry this file is no longer available"
or "this file has been deleted from your drive, would you like to recover it?"
or something like that.
This is very important as there is nothing worse than wasting hours blindly working on an out of date game file.
Yes I have done this a few times it is not fun especially as it usualy takes a day or two to realize