Oh for people asking about saving worlds- there is third party (free) software you can download onto a pc / desktop computer. Im horrible w technology and a couple easy YouTube videos did the trick- can copy worlds onto the pc and even move them onto different devices. If i can do it you can!
I am experiencing a similar problem. Whenever I turn off my iPhone 6S and turn it back on and open Minecraft PE it will show the Mojang Screen for a few seconds before crashing. I am unable to fix this besides deleting Minecraft and re-downloading it but I lose all my worlds, which I spent 3 years on.
it will function normally again after this but once i turn it off overnight it stops working again.