The bug

Trying to recreate a 1.13 world in 1.12 causes crashes and unexpected behavior. Expected would be that exceptions are caught and that a warning is displayed, similar to the one which appears when opening a world created in a future version.

This report is completely valid because it does not require any game downgrading.

Related to MC-123218?

How to reproduce

1. Create a world in 17w50a or a newer version
2. Start the launcher in 1.12.2 release, re-create the world that you just created in a future version
   → Your game will crash.
   
   
3. Start the launcher in 1.12.2, notice that, although you just crashed, the game did re-create that world for you.
4. Enter the re-created world.
   → Your game will crash.
   

Note: The crash as described seems to occur with Void worlds.
I don't get the exact crash outcome if I test it with other premade customized worlds.

Note the "Block being updated" referring to "tile.cocoa".