problem with a pop-up window

Hi all! I've got a problem getting this code to work. The "main menu" is built the way it should be but the pop up window that's expected to appear when "options" button is clicked, does not show up. is there something wrong with my code?

 public class Main_menu : MonoBehaviour {
     
     public Rect windowRect_p =  new Rect ((Screen.width-140)/2-30,(Screen.height-30)/2-30,200,210);
     
     void OnGUI() {
         
         GUI.Box (new Rect ((Screen.width-140)/2-30,(Screen.height-30)/2-30,200,210), "Main Menu");
 
 if (GUI.Button (new Rect ((Screen.width-140)/2, (Screen.height-30)/2+90, 140, 30), "Options")) {
             
             windowRect_p = GUI.Window(0, windowRect_p, perform_Window, "Options");
         }
     }
 
 void perform_Window(int windowID) {
         if(windowID==0){
             if (GUI.Button(new Rect(10, 20, 100, 20), "Sounds"))
             ...;
         
       
             }
     }