[Space Shooter Tutorial] NullReferenceException: Object reference not set to an instance of an object PlayerController.FixedUpdate ()

Hey Guys,

i'm completely new to unity and started a few days ago with the "Roll a Ball" tutorial. Now I'm stuck with the Space Shooter tutorial, I hope you can help. My problem is the following error:

NullReferenceException: Object reference not set to an instance of an object PlayerController.FixedUpdate () (at Assets/Scripts/PlayerController.cs:31)

I've got plenty errors before, but were able to fix them, but this one is too hard for me. Here is my code:

 using UnityEngine;
 using System.Collections;
 
 
 public class Boundary
 {
     public float xMin, xMax, zMin, zMax;
 }
 
 public class PlayerController : MonoBehaviour
 {
     public float speed;
     public float tilt;
     public Boundary boundary;
     public Rigidbody rb;
     
     void Start(){
         
         rb = GetComponent<Rigidbody> ();     //The new shape to push an information from Rigidbody. I think.
         
     }
     
     void FixedUpdate ()
     {
         float moveHorizontal = Input.GetAxis ("Horizontal");
         float moveVertical = Input.GetAxis ("Vertical");
         
         Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
         rb.velocity = movement * speed;
         
         rb.position = new Vector3 (
                 Mathf.Clamp (rb.position.x, boundary.xMin, boundary.xMax), 
                 0.0f, 
                 Mathf.Clamp (rb.position.z, boundary.zMin, boundary.zMax)
             );
         
         rb.rotation = Quaternion.Euler (0.0f, 0.0f, rb.velocity.x * -tilt);
     }
 }

I copied the code from the YouTube comments because it was the actual, working version of the older code used in the video, but my original code looked pretty much the same with a few exceptions.

Link to the video: https://www.youtube.com/watch?v=rVSLczG1M1E