"TypeError: this.treeBox is null" Rated 4 out of 5 stars

This is the same to me:
i get this popup (TypeError: this.treeBox is null) everytime i open a post from my feeds, what is it and how can i get rid of it?
x640238 (not registered):
I had this problem caused by the extension "feed sidebar". Disabled it and the problem went away."

This review is for a previous version of the add-on (4.1).