Rosethorn Acolyte cancel button (for mana ability) not working.

I tried to select Rosethorn Acolyte to block with (but clicked it too early, was still during the declare attackers step). I have the setting set where when there is only one possible action, it goes automatically to that action - so I got the popup asking me to select a colour for the mana. There was a cancel button - but it was greyed out and unusable. Sorry no screenshot.