Prevent (Entity) null to be added to the entity list.

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

@@ -866,6 +866,8 @@ public class World implements IBlockAccess {
 
     public boolean addEntity(Entity entity, SpawnReason spawnReason) { // Changed signature, added SpawnReason
         // CraftBukkit end
+        if (entity == null) return false;
+
         int i = MathHelper.floor(entity.locX / 16.0D);
         int j = MathHelper.floor(entity.locZ / 16.0D);
         boolean flag = false;