[plt-scheme] bytes->path: path string contains a null character -- internal error

I've today encountered an internal error message:

  bytes->path: path string contains a null character:

(followed by the path) which is displayed upon opening 
drscheme (301, windows) when I attempt to drop down 
any menu.  

Can anyone suggest how to reset the path, or otherwise 
how to clear the error?  

This may have been triggered by my replacing a file
which was open in drscheme.  

Thank you!

Doug