Minecraft v1.15.2
Forge v31.1.30
World Blender v1.3.2

I got an error on load that says "World Blender encountered an error during the complete event phase: null"
Log: Error starts at[16:46:38]

Bugs already?! AAAAAAAAAAAAAAAAA!!!

Now that I got that out of my system lol, I think I know what the problem is and how to fix it. Try this quick fix jar and see let me know if it fixes the issue for you. :)
world_blender-1.15.2-1.3.2.zip

Yep, that fixed it, thanks!

Perfect! Thank you for reporting this issue!