This happens on the preview screen when adding a new seed or generating a random one.

Complete log and modlist available on mclo.gs

Thank you!

Thank you for the report!
Could you try to provide us a pack of the mods you are using?
I was unable to reproduce the error in the base forge version.

I have included a BETA version that includes a fix for the error in the log, though I am unsure if there are further errors past that.
world_preview-1.0.1-BETA.zip

Should now also be fixed with the latest release.