I get an issue after instantiating and assigning a variable game object

I have a main menu. When you click a button it changes to a new terrain/water/land. I destroy the old game object (with everything in it) and create a new one. Inside the new one are tags 'water', 'terrain' and 'land'. It is verified the tags are there and correctly addressed.

When I load the scene, the FIRST time the game runs, it correctly displays the water/terrain/land. However, if I try to turn any of those off, it gives and error. In the inspector, the public variable is correct.

When I press the button, it should destroy the old game object, load the new one in with tags, look at the tags and assign them so that they can be turn on or off. It is not doing this. I try to assign the tags after the new object is instantiated but sometimes it does not assign correctly. As I said, the FIRST time it assigns properly and after that it does not.

I am thinking that after instantiating the new object, the assign of the tag should work. it doesn't seem to be doing that. (the first part is repeated 6 times foe each of the game objects to load. They are the same) Any ideas?

 public class MainMenu : MonoBehaviour
 {
     //////////////////////////////////////////////////////////////////////////////////////
     Settings settingsClass;                 // script reference
     //////////////////////////////////////////////////////////////////////////////////////
       public GameObject land;
     public GameObject water;
     public GameObject mountainsObject;
     public Terrain mountainsEmpty;
   void Start()
     {
            ChangeTerrain();                              
            ChangeEnvironment();
 //////////////////////////////////////////////////////////////////////////////////////
      public void ChangeTerrain()
     {
         if (Set.nameOfCourseInt == 1)
         {
             if (GameObject.FindWithTag(Set.nameOfCourseStr1))
             {
                 ////////////////////////////////////////////////
                 //////////
                 //Rockbania Flats
                 //////////
                 if (GameObject.FindWithTag("RockbaniaS"))
                 {
                     Destroy(GameObject.FindWithTag("RockbaniaS"));
                 }
                 ////
                 if (GameObject.FindWithTag("Volcania"))
                 {
                     Destroy(GameObject.FindWithTag("Volcania"));
                 }
                 ////
                 if (GameObject.FindWithTag("Greenlandia"))
                 {
                     Destroy(GameObject.FindWithTag("Greenlandia"));
                 }
                 ////
                 if (GameObject.FindWithTag("Icelandia"))
                 {
                     Destroy(GameObject.FindWithTag("Icelandia"));
                 }
                 
                 ////////////////////////////////////////////////
                 course1Settings.SetActive(true);    //Flats of Rockbania
                 course2Settings.SetActive(false);   //Slopes of Rockbania
 
                 course3Settings.SetActive(false);   //Volcania
                 course4Settings.SetActive(false);   //Greelandia
                 course5Settings.SetActive(false);   //Icelandia
 
                 settingsClass = GameObject.FindWithTag(Set.nameOfCourseStr1).GetComponent<Settings>();     // get script
                 settingsClass.ResetSettings();                                                             // reset game settings
 
                 text1.text = "Flats of Rockbania";
                 ////
                 if (!GameObject.FindWithTag("RockbaniaF"))
                 {
                     Instantiate(courseRockbaniaF, Vector3.zero, Quaternion.identity);           
                 }
             }
         }
         ////////////////////////////////////////////////
         else if (Set.nameOfCourseInt == 2)
         {
             if (GameObject.FindWithTag(Set.nameOfCourseStr2))
             {
                 ////////////////////////////////////////////////
                 if (GameObject.FindWithTag("RockbaniaF"))
                 {
                     Destroy(GameObject.FindWithTag("RockbaniaF"));
                 }
                 //////////
                 //Rockbania Slopes
                 //////////
                 if (GameObject.FindWithTag("Volcania"))
                 {
                     Destroy(GameObject.FindWithTag("Volcania"));
                 }
                 ////
                 if (GameObject.FindWithTag("Greenlandia"))
                 {
                     Destroy(GameObject.FindWithTag("Greenlandia"));
                 }
                 ////
                 if (GameObject.FindWithTag("Icelandia"))
                 {
                     Destroy(GameObject.FindWithTag("Icelandia"));
                 }                
                 ////////////////////////////////////////////////
                 course1Settings.SetActive(false);
                 course2Settings.SetActive(true);
 
                 course3Settings.SetActive(false);
                 course4Settings.SetActive(false);
                 course5Settings.SetActive(false);   //Icelandia
 
                 settingsClass = GameObject.FindWithTag(Set.nameOfCourseStr2).GetComponent<Settings>();     // get script
                 settingsClass.ResetSettings();                                                             // reset game settings
 
                 text1.text = "Slopes of Rockbania";
                 ////
                 if (!GameObject.FindWithTag("RockbaniaS"))
                 {
                     Instantiate(courseRockbaniaS, Vector3.zero, Quaternion.identity);
                 }
             }
         }
         ////////////////////////////////////////////////
                    /////
         //Set objects from tags
             land = GameObject.FindWithTag("land");
             water = GameObject.FindWithTag("water");
             mountainsObject = GameObject.FindWithTag("terrain");
         /////
     }
     /////////
     public void ChangeEnvironment()
     {
         /////
             Debug.Log(environmentInt);
             Debug.Log(land);
             Debug.Log(water);
             Debug.Log(mountainsObject);
         ////////////////////////////////////////////////
         switch (environmentInt)
         {
             case 4: //water on, mountains on     
                 land.gameObject.SetActive(false);
                 water.gameObject.SetActive(true);
                 mountainsObject.gameObject.SetActive(true);
                 mountainsEmpty = mountainsObject.GetComponent<Terrain>();
                 mountainsEmpty.enabled = true;
 
                 text2.text = "Water/Lava & Mountains";
 
                 break;
 
             case 1: //land only
                 land.gameObject.SetActive(true);
                 water.gameObject.SetActive(false);
                 mountainsObject.gameObject.SetActive(false);
                 mountainsEmpty = mountainsObject.GetComponent<Terrain>();
                 mountainsEmpty.enabled = false;
 
                 text2.text = "Land Only";
 
                 break;
 
             case 2:  //water/lava only
                 land.gameObject.SetActive(false);
                 water.gameObject.SetActive(true);
                 mountainsObject.gameObject.SetActive(false);
                 mountainsEmpty = mountainsObject.GetComponent<Terrain>();
                 mountainsEmpty.enabled = false;
 
                 text2.text = "Water/Lava Only";
 
                 break;
 
             case 3:  //Land on, mountans on
                 land.gameObject.SetActive(true);
                 water.gameObject.SetActive(false);
                 mountainsObject.gameObject.SetActive(true);
                 mountainsEmpty = mountainsObject.GetComponent<Terrain>();
                 mountainsEmpty.enabled = true;
 
                 text2.text = "Land & Mountains";
 
                 break;
 
             default: //water on, mountains on
                 environmentInt = 4;
                 land.gameObject.SetActive(false);
                 water.gameObject.SetActive(true);
                 mountainsObject.gameObject.SetActive(true);
                 mountainsEmpty = mountainsObject.GetComponent<Terrain>();
                 mountainsEmpty.enabled = true;
 
                 text2.text = "Water/Lava & Mountains";
 
                 break;
         }
     }
     /////////

 //////////////////////////////////////////////////////////////////////////////////////
     //p2 Change Course Button
     public void OnCourseBtn()
     {
         //////////
         audio2.PlayOneShot(menuSound10, 1f);
         //////////////////////////////////////////////////////////////////////////////////////
         Set.nameOfCourseInt = Set.nameOfCourseInt + 1;    //First increment course name by 1        
 
         if (Set.nameOfCourseInt < 1)
         {
             Set.nameOfCourseInt = 1;
         }
         //////////
         if (Set.nameOfCourseInt > 5)
         {
             Set.nameOfCourseInt = 1;
         }
 
         text2 = courseText.GetComponent<Text>();
         //////////////////////////////////////////////////////////////////////////////////////
         if (Set.nameOfCourseInt == 1)
         {
             course1Settings.SetActive(true);
             course2Settings.SetActive(false);
 
             course3Settings.SetActive(false);
             course4Settings.SetActive(false);
             course5Settings.SetActive(false);   //Icelandia
 
             Set.nameOfCourseStr = "settings-1";
 
             settingsClass = GameObject.FindWithTag(Set.nameOfCourseStr).GetComponent<Settings>();         // get script
             settingsClass.ResetSettings();                                                                 // reset game settings
 
             text2.text = "Flats of Rockbania";
 
         }
         //////////////////////////////////////////////////////////////////////////////////////
         else if (Set.nameOfCourseInt == 2)
         {
 //////////////////////////////////////////////////////////////////////////////////////
     //p2 ENVIRONMENT Button
     public void OnEnvironmentBtn()
     {
         ///////////////////////////////////////////
             audio2.PlayOneShot(menuSound10, 1f);
         ///////////////////////////////////////////
         //Sets Envirnonment Up with name
             text3 = environmentText.GetComponent<Text>();
         ///////////////////////////////////////////
             environmentInt = environmentInt + 1;
         ///////////////////////////////////////////
             if (environmentInt < 1 || environmentInt > 4)
             {
                 environmentInt = 1;
             }
         ///////////////////////////////////////////
             ChangeEnvironment();
         ///////////////////////////////////////////
         // Save to disk
          ES2.Save(environmentInt, "keyEnvironmentInt");
     }
     //////////////////////////////////////////////////////////////////////////////////////