I created a spawner that spawns zombie villagers that are being cured. When I open the world in minecraft, it crashes with an NPE.
After investigating it in MCP, I found that it's caused by a zombie villager model in the spawner. It uses a dummy entity, with world (a.k.a. level) being set to null. But when a zombie starts curing, it accesses the world. A simple if (world != null) should fix it.