Thanks for reporting this bug Illustrator Team really appreciate this.
We are able to reproduce this issue in house and we are investigating it .
An error occurred while saving the commentMatthew Cornier commented
This problem can occur when you have a solo object that is grouped by itself for some reason.
I went into the various group isolations to see when it would access the annotator, it turned out the object I was editing the gradient of was grouped by itself after ungrouping the annotator appeared outside the isolation
hope this helps I've struggled with this problem before and only now have i figured that out.