Thanks for looking into this! I tried and the error message goes away. I still encounter a
problem: once rgrep asks for the file extensions, icomplete accepts any input that is not in
the completion list (e.g. abcdf*.sdf <Enter> or simply the default) but fido says
"incomplete" and does not allow to proceed unless I select a directory / file from
the completion list (which does not really make sense; C-f to change to the default prompt like
with ido in find file does not work). This might be an error on my side though not knowing the
right shortcut - I am still new to ido / fido (sorry!).