Put the summary of the bug you're having here

What I expected to happen was...:
Expected normal startup when updating existing world from w21b.

What actually happened was...:
screen flashed comment about server being down, although not using server, just home computer. Then screen changed to say game crashed.

Steps to Reproduce:
1. Start Minecraft
2. Select 18w22a
3. Select single player

4. Select existing world, select play