Did Mr. Trump need to cancel the event? Not according to the Chicago PD.
Did Mr. Trump cancel the event simply in order to get attention? Your comparison to a father of the bride having to cancel a wedding because of some disruptive religious bigots doesn't fly. Donald Trump can afford to write thousands of checks for what he paid in rent for the event facility. He's not going to miss that money as much as much as a middle class father of the bride is going to miss what he shelled out. Moreover, Mr. Trump is attempting to put the blame on Senator Sanders and his campaign for the disruption? That's a charge any rational person would know is nonsense. Is Mr. Trump trying to financially cripple the organization of a potential future opponent? I don't know, but it's a question worth asking.
What this appears to be is a spontaneous mob, which no doubt included many who plan to vote for Senator Sanders today, protesting a speaking engagement by a hatemonger. How does one sue a mob to recover losses? One might sue the city of Chicago for not providing adequate security, but even that is questionable at this point.
Mr. Trump can afford to eat the loss for canceling the event, but he'll get his money's worth trying to blame Senator Sanders, a candidate he consistently mischaracterizes as a "communist." If Mr. Trump chooses to file a suit against the Sanders campaign, it will go nowhere. Otherwise, Mr. Trump is going right ahead and being something he is everyday, a demagogue.