If you change out an existing resource pack with a new one and open a world the game will crash.

Steps to reproduce

• Add 2 texture packs to your resource_packs folder. (or buy the city and plastic texture packs but close your game after you do.)
• Open your game.
• Press PLAY then go back to title screen.
• Go to your texture packs page. Options> Texture packs > Manage
• Your game will sometimes decide to select a custom texture pack by default. (If it does, change it out with the one you want to use.)
• Change out current texture pack being used and pick a new one. (skip this step if there isn't anything added.)
• Add the texture pack you want to use. (skip this step if you already have a texture pack added.)
• Now open one of your worlds and your game will crash.