Set world on fixed tile entity to avoid NPE. Addresses BUKKIT-3949

src/main/java/net/minecraft/server/WorldServer.java

@@ -139,6 +139,7 @@ public class WorldServer extends World implements org.bukkit.BlockChangeDelegate
 
         if (Block.byId[type] instanceof BlockContainer) {
             TileEntity replacement = ((BlockContainer) Block.byId[type]).b(this);
+            replacement.world = this;
             this.setTileEntity(x, y, z, replacement);
             return replacement;
         } else {