PaintTool Sai + League = comp lock up

I've encountered this bug multiple times and I'm not sure if it's directly related to League or if its the combination of the two and my computer. But when I have paint tool sai open, and am working on something while in the game lobby and the game starts, on occasion I will be completely locked out of everything.

The screen will be open to the loading screen, and even load into the game like usual, but I am locked out of clicking on anything, or even tabbing to another program. The only thing I can do at that point is shut my comp off from the tower and hope I can get back into game before they remake.