My game has literally just updated and the window has gone! I'm so happy to be able to play my favourite mobile game again! (I hope it doesn't happen again though!)
My phone updates are up to date, tried force stop on Pet Rescue app but it had no effect. I am synced with Facebook but I wasn't trying to connect at the time. (I had previously been playing Candy Crush but the Facebook syncing made that app crash. It's fine now though)