Texture2D Extension: Texture Still Null After Calling It

Your best bet is to use a plain old static method with a ref parameter.

 public static class ResetTexture {
   public static void Reset(ref Texture2D tex) {
     if(tex == null) tex = new Texture2D(256,1,TextureFormat.RGBA32,false);
     for(int i=0;i<256;i++) {
       tex.SetPixel(i,1,Color.clear);
     }
     tex.Apply();
     tex.wrapMode = TextureWrapMode.Clamp;
   }
 }
 
 // Somewhere else
 if (ref_tex == null) ResetTexture.Reset(ref ref_tex);
 ref_tex.SetPixel(i,1,new Color(r,g,b,1f));

The reason you can't use an extension method is due to how parameters are passed in C#. Parameters are by default passed by-value. Changing the parameter using = inside the method has no effect on the variable passed to the method.

 void Start() {
   var a = "Hi";
   Debug.Log(a);
   Change(a);
   Debug.Log(a);
 }
 
 void Change(string b) {
   Debug.Log(b);
   b = "Hello";
   Debug.Log(b);
 }

Produces output:
Hi
Hi
Hello
Hi

When you want changes inside a method to propagate back to the caller you use the ref keyword.

 void Start() {
   var a = "Hi";
   Debug.Log(a);
   Change(ref a);
   Debug.Log(a);
 }
 
 void Change(ref string b) {
   Debug.Log(b);
   b = "Hello";
   Debug.Log(b);
 }

Produces output:
Hi
Hi
Hello
Hello

However, the first parameter of an extension method can't be a ref parameter.

 void Change(this ref Texture2D b) {
   // Compile Error: "The parameter modifiers 'this' and 'ref' cannot be used together."
 }

This means you can't use ref to modify the object on which you call an extension method.