First, it is a bad idea to set up a while loop inside of an event case. An event case should return quickly from whatever it is doing so that the event structure can be ready to handle the next event. A while loop inside prevents that.
Your main problem is that if you right click on the event case and edit its properties, down to the lower left, you'll see you have a checkbox checked that says Lock front panel until this event completes. You've created a catch 22, the event case can't complete until you stop that inner while loop by way of the stop button, but you can't hit the stop button because the front panel has been locked.
Uncheck that box and you will find things working better.
But in reality, the stop button should have its own event case to handle its button press, and you shouldn't have any code inside an event case like a while loop or user dialog that blocks the execution of the event structure for a long period of time.
