Adding RequireComponent breaks script

After adding a RequireComponent directive to this script:

 @script RequireComponent (Window)
 
 class Menu extends MonoBehaviour{
     private var window : Window;
     
     function Start(){
         window = GetComponent(Window);
     }
     
     function OnGUI(){        
         Main.INSTANCE.executeSomeFunction();
     }
 }

I can no longer access one of my singleton scripts. I get the error message: 'INSTANCE' is not a member of 'function(): void'.. Here is the singleton class:

 class Main extends MonoBehaviour{
     static var INSTANCE : Main;
 
     function Start(){
         INSTANCE = FindObjectOfType(Main);
         setState(initialState);
     }
     
     function OnApplicationQuit(){
         INSTANCE = null;
     }
 }

Why is this happening?