Cant change a float value in a Shader Graph

Hi guys as the title states, I am having trouble changing a float value in an Outline shader Graph.
The shader graph works perfect and I want to control it through a c# sharp script that is attached to my main player object.
I have created a material with the shader graph and attached it to my main player

Code (CSharp):

1.  Ship = GetComponent<Rigidbody2D>();
2.         animator = GetComponent<Animator>();
3.         gameController = FindObjectOfType<GameController>();//find GameController script might be better to do this rather than making the vairables static?
4.         spawner = FindObjectOfType<WaveSpawner>();
5.         audioSource = GetComponent<AudioSource>();
6.         spriteRenderer = GetComponent<SpriteRenderer>();
7.         gameController.PlayerAlive = true;
8.         spawner.waveSpawner = true;
9.         stealth(3f);
10.         powerUp = 0;
11.         defaultColour = new Color32(0, 97, 255, 255);//set colours
12.         selectedColour = new Color32(252, 12, 132, 255);
13.         maxColour = new Color32(0, 0, 0, 255);
14.         inUseColour = new Color32(127, 233, 255, 255);
15.         inVincibleColour = new Color32(127, 233, 255, 255);
16.         hurtColour = new Color32(252, 12, 96, 255);
17.         playerColour = new Color32(176, 88, 214, 255);
18.  
19.        // playerMaterial = GetComponent<SpriteRenderer>().material;
20.         playerMaterial = spriteRenderer.material;

I then get the sprite renderer component in start
Then I want to change the _Thickness property, in my stealth method

Code (CSharp):

1.     public void stealth(float stealthTime) //pass in time for stealth
2.     {
3.         timer = stealthTime;
4.  
5.         if (weaponMax[gameController.t] != true)
6.         {
7.             lineThickness = 0.005f;
8.             playerMaterial.SetFloat("_Thickness", lineThickness);
9.  
10.             isInvicilbe = true;
11.             weaponMax[gameController.t] = true;
12.             gameController.UItext[gameController.t].color = maxColour;
13.                                                                  
14.  
15.             spriteRenderer.color = new Color32(127, 233, 255, 255);
16.          
17.         }

I am getting a
NullReferenceException: Object reference not set to an instance of an object
PlayerController.stealth (System.Single stealthTime) (at Assets/Scripts/PlayerController.cs:339)
PlayerController.Start () (at Assets/Scripts/PlayerController.cs:72)

 

Ok Thanks will try there

 

If you could that would be great thanks @MelvMay