Assertion failed: Assertion failed on expression: 'IsActive() || GetRunInEditMode()' What does this mean?

I am using the code below and although it works perfectly i get this error whenever i press the button on my keyboard. Is this a bug of the engine or something wrong with my code?

 private void OnGUI()
     {
         if (pistol_button)
         {
             if(GUI.Button(new Rect(20, Screen.height - 50, 100, 50), "1. Pistol") || Input.GetKeyDown(KeyCode.Alpha1))
             {
                 child1.SetActive(true);
                 child2.SetActive(false);
             }
         }
         if (shotgun_button)
         {
             if(GUI.Button(new Rect(140, Screen.height - 50, 100, 50), "2. Shotgun") || Input.GetKeyDown(KeyCode.Alpha2))
             {
                 child2.SetActive(true);
                 child1.SetActive(false);
             }
         }
     }