indirectly encouraging the less than civil behavior of some of his followers with statements he has made combined with his lack of willingness to directly and unequivocally denounce violence.

I guess at first glance, this one didn't strike me as being that bad. The term "riot" can be literal or figurative. But you're right, given the lack of civility a portion of his supporters have shown to date, it's ill-advised (and likely purposefully done) for him to make any reference to "riots" if he is denied the nomination.