Grey frames in Lightroom catalog cannot be deleted - error message received

I have several grey frames in my Lightroom catalog and I cannot delte them.  The error message is as follows

An internal error has occured:?:0: attempt to index field 'rootFile' (a nil value)

I run a PC with the latest Windows software.  I have updated to the latest Lightroom version and have made changes to color management as seen in a previous post.

Nothing seems to work.