How to send messages to a window

I've got a hung window "Set Title" in Ubuntu mate-terminal that doesn't react to input. I can move the window, but it doesn't render properly (with text box and "Ok" button), nor can I click the window icon on the top left. I can click the [x] on the top right, but nothing happens for a few seconds until I get a dialog asking whether I want to kill the unresponsive application. I don't want that since it'll terminate all my mate-terminal instances which would make me very sad. I can still spawn new terminals. Is it possible to send that window a message (WM_CLOSE?) to just go away as if the ESC key had been pressed, and if so, how (I know xprop, but that's just for displaying window properties)? Bonus points if I can do that on the command line, but compiling a simply C program would be ok too. Thanks.