Getting error once the comment is flaged a solved in forum

There is no green box around the solved comment as shown in the example.
and
This error pops up after I flag solved a topic in a forum based on a comment. The same user who created the topic flags his own solution which he published as a comment.

Notice: Undefined index: classes in flag_solved_preprocess_comment() (line 185 ... \sites\all\modules\flag_solved\flag_solved.module).

thanks in advance for any help.