Issue with adding a new column in PowerBI Dataflow

I created a Dataflow in PowerBI Service. I was trying to create a new column to one of the tables. When I set the Column header as blank and tried to cancel it, I got the error message box. But when I press "Close" button on the box, it does not close and the message box stays there no matter how many times I press the button. I then quickly had to press a character before I was finallly able to close the box.