If you click twice on the DAX Studio button, multiple windows of DAX Studio are opened. However, if you:
1) Open first DAX Studio window
2) Run a query on the first window
3) Open second DAX Studio window
4) Run a query on the second window
5) Run a query again on the first window
 
The AddIn crashes. I think it would be better having just one DAX Studio window open - we can open multiple excel instances or handling multiple queries in a future release, but now it's safer to handle just a single window.