My gameObject starts flashing

When I run my project, my gameObject starts flashing. In the inspector, the tick of the checkBox of my object changes. What I'm doing is setting all my GameObjects inactive since the beggining, and while the project is running, I begin to set active all my objects.

My code:

     public bool isSecondPage = false;
     public bool isThirdPage = false;
     public bool isFourthPage = false;
     public bool isFifthPage = false;
     
     
     void Update () {
         
         if(isSecondPage==true){
             cuadro1.SetActive(true);
             cuadro2.SetActive(true);
 
         }