I am playing a world and it just suddenly crashes or stops with no warning.

I was playing a world with a treehouse in that I was making and I tried to change my skin and the game just crashed. I tried again and nothing happened. I tried to rename a pig and it crashed again. Again, I tried again and it was fine. I was messing around with a few commandblocks and it crashed again. Eventually, I spotted a village in the distance and tried to go towards it. The game stopped with no warning. Can someone please explain all these crashes and stops?

Today I had another crash on my game. I was playing the template parkour paradise and I was just about to hit the ground and make the jump when the game froze. And crashed. Was it that I can't save and quit if I don't want the game to crash?