Try this: whenever you shutting off your Game, Charge it right away, and stay close to your WI-FI signal while youre playing.
Believe Me!
I had the same problem. My game was always crashing when I was joining a server. I tried this, and it worked!
If this doesnt solve youre problem, try charging your device while youre playing.
(If it still doesnt work, charge and stay close to your WI-FI signal.)
Hope this solves your problem
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HalfRobot200 about Minecraft: Pocket Edition, v1.1.5