[CLIENT] An unclosable "White Screen" appeared in champ select when i opened and closed masteries

I was in a ranked champ select trying to tune my mastery page, when i attempted to close it, the screen turned white and refused to close. It blocked my champion grid pane and led to my unintentional dodging of the game. I tried re-sizing the client so that I could select a champ, but on all measurements champion grid pane was totally obscured by it. I am attaching a screenshot link, but it is of moment after i got out of the champ select (you can see it in event log). I cannot say if I can reproduce the bug because I wouldn't take the risk of getting punished. I have faced it only once so far. Screenshot of the screen