Tree thingie

Please help. I made a new tree type for a new nether biome, but it won't generate in the nether (trees don't do that I know). So I made my tree a structure, and made a function that should spawn them in the biome. But now, when I open my minecraft and click create new world, minecraft says saving world and closes. Why does he do that and how do I stop that. Is it a crash or something else (I think its a crash, I have a bad laptop but I REALLY WANT TO TEST MY MOD)