Function call from one class to another not working.

Hi, I have a function in class1 which I try to call from another class2:

Code in class1:

 function AddCategoryListener (button: Button) {
     button.onClick.AddListener( function() {DoSomething(button);} );
 }
         
 function DoSomething (button: Button) {
     //...
 }

Code in class2:

 GetComponent (class1).AddCategoryListener (a.GetComponent (Button));

At runtime, when the code from class 2 is being executed, I get a NullReferenceException in the last line of class 2.

But if I add the following infront of the last line it never prints "null":

 print (class1);
 print (a);
 print (a).GetComponent (Button));

Could anybody help me with that?