App crashes apon playing and opening app

When I open the app alotta times it crashes and goes back to the main screen of the phone,  also other times I open the app and begain playing and it crashes and goes back to the main screen. I see mobs like sheep, I go over to them and if the mob went down a block as soon as I get there the mob despawned, as if there was a hole there that the mob fell down in I get there and it's just sand or dirt there or where ever it happend, other times I open the  app I open a world and join it, then it crashes and loads world and then I can play it without it crashing.