For some reason, MixinStructureGeneratorBaseMM is not working properly and the game still crashes.
Although your Overwrite generateLayer looks like it should work, maybe there is some another thing that could be null? (It's unlikely to say this for sure, because in StructureGeneratorBase.java there are generally only 565 lines, and the report indicates line 742. which obviously doesn't exist. And the second line in the report - 380, contains only the use of the generateLayer method)
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Help isloating the issue
Game log
https://mclo.gs/PwotTOn
crash report: https://mclo.gs/E1eV7ux
Description
For some reason, MixinStructureGeneratorBaseMM is not working properly and the game still crashes. Although your Overwrite generateLayer looks like it should work, maybe there is some another thing that could be null? (It's unlikely to say this for sure, because in StructureGeneratorBase.java there are generally only 565 lines, and the report indicates line 742. which obviously doesn't exist. And the second line in the report - 380, contains only the use of the generateLayer method)