Graphics glitch and unable to save the game

Every so often the graphics on my game will glitch until most blocks are are invisible... if this happens portals and commands don’t work. It also makes it impossible to exit the game by pressing the pause game. It brings up a loading pop up similar to the main loading screen. 
In order to exit the game you have to exit and close the app manually and when you renter the game and your world it takes you back to the most previous save.