When selecting world, world seems to load as usual, get to the terrain loading on first try, but then crashes.

On a second attempt, gets as far as just opening the world (I could see my world.structures for a moment) then game crashes. 

On another attempt, that world was no longer in my menu of available worlds.

Checked saves folder on my computer. World folder/save is still there.

Changed my launcher to re-download and play 1.13-pre8. At first world still not visible in list, but then retrieved game-save folder from back-ups folder and copied into the saves folder (renaming the original).  Then, was able to see both the rename and backed-up saves in my available worlds.

Was able to play the world (using renamed game-save) in 1.13-pre8.