Enforcing contractual obligations isn’t unconstitutional. We don’t know what the contract actually says but it might plausibly include damages for cancelling in these circumstances.

Edit: With that said, the center isn’t helping its case by claiming things like

Your dismal ticket sales and lack of donor support, combined with your last-minute cancellation has cost us considerably. This is your official notice that we will seek $1 million in damages from you for this political stunt.

Calling the action a political stunt may allow the defendant to argue that the lawsuit is motivated by retaliation against speech even if the lawsuit would otherwise be valid, and admitting that ticket sales were dismal isn’t great if you’re going to seek damages for lost ticket sales.

“Controversial” here seems like a basic statement of fact. One might think that she ought not to be controversial, but she is.

That’s not how it would work even for an ordinary person. This is an accusation about something rather far-fetched happening 35 years before the accusation was made, with no evidence for it except the accusation itself. It wouldn’t lead to criminal charges.