Tried to make a super flat world with glass as the top layer and bedrock below, and a village which is default, after i edited the layers and clicked on use preset, the screen changed to the loading screen which said "Saving world" but just for a split second and then crashed.

Here is the crash report.

 
The game crashed whilst mouseclicked event handler
Error: n: Non [a-z0-9/._-] character in path of location: minecraft:glass,minecraft:plains
 

I tried this twice, and it still did not works so I switched up the layers to make bedrock on top and glass on the bottom and it worked... Not sure why exactly this happens.