Put the summary of the bug you're having here
In world selection choose a 1.16 world, click EDIT, click OPTIMIZE WORLD, click I KNOW WHAT I'M DOING

What I expected to happen was...:
The optimize world screen would appear and the game would "do it's thing"

What actually happened was...:
The game crashed

Steps to Reproduce:
1. I had created a test 1.16 world when the first snapshot was available
2. After starting the game under 20w17a I noticed a beacon was not working
3. The beacon worked once I broke and replaced the beacon block
4. >>> I do not know if the above has anything to do with this bug <<<
5. I saved the game and restarted it to make sure the beacon was still working – it was
6. I thought that perhaps there were some changes in 20w17a that might require any old data to be updated, so...
7. I saved the game
8. From the PLAY menu, I selected this world again
9. I clicked EDIT
10. I clicked OPTIMIZE WORLD
11. I clicked "I KNOW WHAT I'M DOING"
12. The game crashed

I tried with ERASE CACHED DATA checked and unchecked. The game crashes in both cases and the crash dump files are essentially the same