Multiple errors on mate-panel replace: Unable to open desktop file

For a couple days now my panel has been disappearing.

I've been getting it back by running mate-panel --replace. The panel returns okay but I have a cascade of errors

 Unable to open desktop file < file> 

Most of these files are obsolete. They were panel items once upon a time but I've since dropped them.

Where is this list kept? When i find that file can I simply delete the obsolete entries or is there something else I must do.