Make stapi get angree (#169)

Author: calmilamsy

station-flattening-v0/src/main/java/net/modificationstation/stationapi/mixin/flattening/CaveGenBaseMixin.java

@@ -20,6 +20,9 @@ public void stationapi_setWorld(World world) {
     @Override
     @Unique
     public World stationapi_getWorld() {
+        if (stationapi_world == null) {
+            throw new RuntimeException("stationapi_world is null, use CaveGenBaseImpl.stationapi_setWorld in your custom ChunkSource constructor to fix.");
+        }
         return stationapi_world;
     }
 }