Is there a way to see the grey squares? I’m on the left - your tutorial looks to be more useful.
You need to have an instance selected in the Preview below.
Thanks! In fact, the problem was a bit more intriguing:
I’ve been ignoring it, every time, like I was told to… 
So I’ve now just deleted it, and everything works as usual.
Do you have a copy of the file with the annoying.notdef, that you could send me?