Unity On Trigger Enter NullReferanceException

NullReferenceException: Object reference not set to an instance of an object
Birdy.OnTriggerEnter (UnityEngine.Collider collision)
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Birdy : MonoBehaviour
{

public float velotown = 1f;
public Rigidbody rb3d;
public bool IsDead;
public GameManager managerGame;


void Update()
{
if (Input.GetMouseButtonDown(0))
if (IsDead == false)
{
rb3d.velocity = Vector3.up * velotown;
}
}
public void OnTriggerEnter(Collider collision)
{
if (collision.gameObject.tag == "Score")
{
managerGame.UpdateScore();
Debug.Log("done");
}
}
}

 

And game is 2d, no sprites models are 3d.

 

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class Birdy : MonoBehaviour
6. {
7.  
8.     public float velotown = 1f;
9.     public Rigidbody rb3d;
10.     public bool IsDead;
11.     public GameManager managerGame;
12.  
13.  
14.     void Update()
15.     {
16.         if (Input.GetMouseButtonDown(0))
17.             if (IsDead == false)
18.             {
19.                 rb3d.velocity = Vector3.up * velotown;
20.             }
21.     }
22.     public void OnTriggerEnter(Collider collision)
23.     {
24.         if (collision.gameObject.tag == "Score")
25.         {
26.             managerGame.UpdateScore();
27.             Debug.Log("done");
28.         }
29.     }
30. }

 

The answer is always the same... ALWAYS. It is the single most common error ever.

Don't waste your life spinning around and round on this error. Instead, learn how to fix it fast... it's EASY!!

Some notes on how to fix a NullReferenceException error in Unity3D
- also known as: Unassigned Reference Exception
- also known as: Missing Reference Exception
- also known as: Object reference not set to an instance of an object

http://plbm.com/?p=221

The basic steps outlined above are:
- Identify what is null
- Identify why it is null
- Fix that.

Expect to see this error a LOT. It's easily the most common thing to do when working. Learn how to fix it rapidly. It's easy. See the above link for more tips.

This is the kind of mindset and thinking process you need to bring to this problem:

https://forum.unity.com/threads/why-do-my-music-ignore-the-sliders.993849/#post-6453695

Step by step, break it down, find the problem.