Null Reference Exception/Unassigned gameobject error

I have had endless Null Reference Exception errors in making the UI for my game, which probably means I have some sort of basic misunderstanding of how Unity C# works. My current problem is that when I try to click on a gameobject (Def1) I want it to affect another gameobject that was previously clicked (listOfLocations[i]). However, this isn't working because I assign which gameobject within GameObject[] listOfLocations was last clicked to an exterior code, which is referenced in the script as gamecontroller. In the method that assigns this, it is working and I am able to print gamecontroller.GetComponent.currentLocationName(). Yet when I click on that other gameobject and run Def1Upgrade method suddenly there's the error "The variable gamecontroller has not been assigned". I don't know why. Help please.

Here's my code: //This creates GameObject for specific location public GameObject gamecontroller; public GameObject da_Location; GameObject[] listOfLocations = new GameObject[24];

 //Player Stuff
 public GameObject Player1;
 public GameObject Player2;

 //Basic Templates for a Location
 public GameObject tem_Un;
 public GameObject tem_Fort;
 public GameObject tem_Min;
 public GameObject tem_Tem;

 //Location Values
 string theName;
 bool isUnsettled;
 bool isFort;
 bool isMine;
 bool isTemple;
 int defLvl;
 int trainLvl;
 int goldLvl;
 int armLvl;
 int devLvl;
 int fervLvl;
 double Health;
 double TrainRate;
 public double WealthGen;
 public double FaithGen;

 //Unsettled Buttons
 public GameObject objChooFor;
 public GameObject objChooMin;
 public GameObject objChooTem;
 public Button chooFor;
 public Button chooMin;
 public Button chooTem;


 // Use this for initialization
 void Start () {
     isUnsettled = true;
     isFort = false;
     isMine = false;
     isTemple = false;
     defLvl=0;
     trainLvl=0;
     foreach (GameObject s in GameObject.FindGameObjectsWithTag("Locationz")){
         for (int i = 0; i < 24; i++) {
             if (listOfLocations [i] == null) {
                 listOfLocations [i] = s;
                 break;
             }
         }
     }

 //These methods set the specific class for each location
 void CchooseFort (){
     tem_Fort.gameObject.SetActive (true);
     for (int i = 0; i < 24; i++) {
         if (listOfLocations [i].name  == gamecontroller.GetComponent<StartGayme>().currentLocationName) {
             listOfLocations[i].GetComponent<LocationCode>().isUnsettled = false;
             listOfLocations[i].GetComponent<LocationCode>().isFort = true;
             listOfLocations [i].GetComponent<LocationCode>().Health = 400;
             listOfLocations [i].GetComponent<LocationCode>().TrainRate = 1;
         }
     }
     tem_Un.gameObject.SetActive (false);
 }

 //Fort Upgrades
 public void Def1Upgrade (){
     for (int i = 0; i < 24; i++) {
         print (theName);
         print (gamecontroller.GetComponent<StartGayme> ().currentLocationName);
         if (listOfLocations [i].name  == gamecontroller.GetComponent<StartGayme>().currentLocationName && defLvl==0) {
             print ("anything");
             listOfLocations [i].GetComponent<LocationCode>().Health += 150;
             listOfLocations [i].GetComponent<LocationCode> ().defLvl = 1;
             Player1.GetComponent<Player>().setWealth(Player1.GetComponent<Player>().getWealth()-250);
             print (listOfLocations [i].name + " has train rate " + listOfLocations [i].GetComponent<LocationCode> ().trainLvl.ToString ());
             print (listOfLocations [i].name + " has defense " + listOfLocations [i].GetComponent<LocationCode> ().defLvl.ToString ());
             //Reduce alpha of Def1 Image OR make Def1 Image grayed-out/darker;
         }
     }
 }

//sets which location gameobject was clicked last and identifies what type it is public void OnMouseDown () { gamecontroller.GetComponent().currentLocationName = this.gameObject.name; theName = gamecontroller.GetComponent ().currentLocationName; //print (gamecontroller.GetComponent().currentLocationName); print (theName); tem_Un.gameObject.SetActive (false); tem_Fort.gameObject.SetActive (false); tem_Min.gameObject.SetActive (false); tem_Tem.gameObject.SetActive (false); if (this.isUnsettled == true) { tem_Un.gameObject.SetActive (true); } if (this.isFort == true) { tem_Fort.gameObject.SetActive (true); } if (this.isMine == true) { tem_Min.gameObject.SetActive (true); } if (this.isTemple == true) { tem_Tem.gameObject.SetActive (true); } } }