Object reference not set to an instance of an object

I am making an RTS and whenever I try to click on a unit, it returns with an error saying

NullReferenceException: Object reference not set to an instance of an object MousePoint.Update () (at Assets/Standard Assets/Scripts/MousePoint.cs:112)

This is my script, please help.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class MousePoint : MonoBehaviour {
 
     #region Class Variables
 
     RaycastHit hit;
 
     public GameObject Target;
     public static GameObject CurrentlySelectedUnit;
     public static ArrayList CurrentlySelectedUnits = new ArrayList();
     public static ArrayList UnitsOnScreen = new ArrayList ();
     public static ArrayList UnitsInDrag = new ArrayList();
 
     private static Vector3 mouseDownPoint;
     private static bool UserIsDragging;
     private static float TimeLimitBeforeDeclareDrag = 1f;
     private static float TimeLeftBeforeDeclareDrag;
     private static Vector2 MouseDragStart;
     private static float clickDragZone = 1.3f;
     private bool FinishedDragOnThisFrame;
 
     //GUI
     public LayerMask SelectMeshLayerMask;
 
     #endregion
     
     // Update is called once per frame
     void Update ()
     {
 
         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 
         if (Physics.Raycast (ray, out hit, Mathf.Infinity)) {
             //Store point at mouse button down
             if (Input.GetMouseButtonDown (0)) {
                 mouseDownPoint = hit.point;
             
             //Mouse drag
                 if (Input.GetMouseButtonDown (0)) 
                 {
                     TimeLeftBeforeDeclareDrag = TimeLimitBeforeDeclareDrag;
                     MouseDragStart = Input.mousePosition;
                 } 
                 else if (Input.GetMouseButton (0)) 
                 {
                     //If user is not dragging, do tests
                     if (!UserIsDragging) 
                     {
                         TimeLeftBeforeDeclareDrag -= Time.deltaTime;
                         if (TimeLeftBeforeDeclareDrag <= 0f) 
                         {
                             //If tests pass true, user is dragging
                             UserIsDragging = true;
                         }
                     }
 
                     //User is dragging, lets compete (GUI)
                     //if(UserIsDragging)
                     //Debug.Log("User is dragging!");
                 } 
                 else if (Input.GetMouseButtonUp (0)) 
                 {
                     TimeLimitBeforeDeclareDrag = 0f;
                     UserIsDragging = false;
                 }
             
             }
 
             if (hit.collider.name == "TerrainMain") {
 
                 //When we click the right mouse button, instantiate target
                 if (Input.GetMouseButtonDown (1)) {
                     GameObject TargetObj = Instantiate (Target, hit.point, Quaternion.identity) as GameObject;
                     TargetObj.name = "Target Instantiated";
 
                 } else if (Input.GetMouseButtonUp (0) && DidUserClickLeftMouse (mouseDownPoint))
                 {
                     if(!CTRLKeysDown())
                         DeselectGameObjectsIfSelected ();
                 }
 
 
 
             } //End of the terrain
 
             else { 
 
                 //Hitting other objects
                 if (Input.GetMouseButtonUp (0) && DidUserClickLeftMouse (mouseDownPoint)) 
                 {
 
                     //Is the user hitting a unit?
                     if (hit.collider.gameObject.GetComponent<Unit>() || hit.collider.gameObject.layer == LayerMask.NameToLayer("SelectMesh"))
                     {
                         Transform UnitGameObject;
                         if (hit.collider.gameObject.layer == LayerMask.NameToLayer ("SelectMesh"))
                             UnitGameObject = hit.collider.transform.parent.transform;
                         else
                             UnitGameObject = hit.collider.transform;
 
                         //Are we selecting a different object?
                         if (!UnitAlreadyInCurrentlySelectedUnits (UnitGameObject.gameObject)) 
                     {
                             
                             //If the CTRL key is not down, remove the rest of the units
                             if (!CTRLKeysDown ())
                                 DeselectGameObjectsIfSelected ();
 
                             GameObject SelectedObj = UnitGameObject.Find("Selected").gameObject;
                             SelectedObj.active = true;
 
                             //Add unit to currently selected units
                             CurrentlySelectedUnits.Add (UnitGameObject.gameObject);
 
                             //Change unit Selected value to true
                             UnitGameObject.gameObject.GetComponent<Unit> ().Selected = true;
 
 
                         } else {
                             //Unit is already in the currently selected units arraylist
                             //Remove the unit
                             if (CTRLKeysDown())
                                 RemoveUnitFromCurrentlySelectedUnits (UnitGameObject.gameObject);
                             else 
                             {
                                 DeselectGameObjectsIfSelected ();
                                 GameObject SelectedObj = UnitGameObject.transform.Find ("Selected").gameObject;
                                 SelectedObj.active = true;
                                 CurrentlySelectedUnits.Add (UnitGameObject.gameObject);
                                 UnitGameObject.gameObject.GetComponent<Unit> ().Selected = true; //Need to show this
                             }
                         }
 
                     } else {
                         //If this object is not a unit
                         if(!CTRLKeysDown())
                             DeselectGameObjectsIfSelected ();
                     }
                 }
             
 
             }
      
         } else {
             
             if (Input.GetMouseButtonDown (0) && DidUserClickLeftMouse(mouseDownPoint))
                 if(!CTRLKeysDown())
                     DeselectGameObjectsIfSelected ();
         }
     }
 
     void LateUpdate()
     {
         
     }
 
     #region Helper function
 
     //Is user dragging, relative to mouse drag start point
     public bool UserDraggingByPosition(Vector2 DragStartPoint, Vector2 NewPoint)
     {
         if(
             (NewPoint.x > DragStartPoint.x + clickDragZone || NewPoint.x < DragStartPoint.x - clickDragZone) ||
             (NewPoint.y > DragStartPoint.y + clickDragZone || NewPoint.y < DragStartPoint.y - clickDragZone) 
             )
             return true;
         else
             return false;
     }
 
     //Check if a user clicked mouse
     public bool DidUserClickLeftMouse(Vector3 hitPoint)
     {
 
          if(
             (mouseDownPoint.x < hitPoint.x + clickDragZone && mouseDownPoint.x > hitPoint.x - clickDragZone) &&
             (mouseDownPoint.y < hitPoint.y + clickDragZone && mouseDownPoint.y > hitPoint.y - clickDragZone) &&
             (mouseDownPoint.z < hitPoint.z + clickDragZone && mouseDownPoint.z > hitPoint.z - clickDragZone)
             )
             return true; else return false;
     }
 
     //Deselects gameobject if selected
     public static void DeselectGameObjectsIfSelected()
     {
         if (CurrentlySelectedUnits.Count > 0) 
         {
             for (int i = 0; i < CurrentlySelectedUnits.Count; i++) 
             {
                 GameObject ArrayListUnit = CurrentlySelectedUnits [i] as GameObject;
                 ArrayListUnit.transform.Find ("Selected").gameObject.active = false;
                 ArrayListUnit.GetComponent<Unit> ().Selected = false;
             }
 
             CurrentlySelectedUnits.Clear();
         }
     }
 
     //Check if a unit is already in the currently selected units arraylist
     public static bool UnitAlreadyInCurrentlySelectedUnits(GameObject Unit)
     {
         if (CurrentlySelectedUnits.Count > 0) {
             for (int i = 0; i < CurrentlySelectedUnits.Count; i++) {
                 GameObject ArrayListUnit = CurrentlySelectedUnits [i] as GameObject;
                 if (ArrayListUnit == Unit)
                     return true;
             }
 
             return false;
         } else 
             return false;
         }
 
     //Remove a unit from the currently selected units arraylist
     public void RemoveUnitFromCurrentlySelectedUnits(GameObject Unit)
     {
         if (CurrentlySelectedUnits.Count > 0) {
             for (int i = 0; i < CurrentlySelectedUnits.Count; i++) {
                 GameObject ArrayListUnit = CurrentlySelectedUnits [i] as GameObject;
                 if (ArrayListUnit == Unit) 
                 {
                     CurrentlySelectedUnits.RemoveAt (i);
                     ArrayListUnit.transform.Find ("Selected").gameObject.active = false;
                 }
             }
 
             return;
         } else 
             return;
     }
 
     //Are the control keys being held dowN?
     public static bool CTRLKeysDown()
     {
         if (Input.GetKey(KeyCode.LeftControl))
             return true;
         else
             return false;
     }
 
     //Check if a unit is within screen space to deal with mouse drag selection
     public static bool UnitWithinScreenSpace(Vector2 UnitScreenPos)
     {
         if(
             (UnitScreenPos.x < Screen.width && UnitScreenPos.y < Screen.height) &&
             (UnitScreenPos.x > 0f && UnitScreenPos.y > 0f)
             )
                 return true;
             else
                 return false;
     }
 
     //Remove unit from screen units UnitsOnScreen arraylist
     public static void RemoveFromOnScreenUnits(GameObject Unit)
     {
         for (int i = 0; i < UnitsOnScreen.Count; i++) {
             GameObject UnitObj = UnitsOnScreen [i] as GameObject;
             if (Unit == UnitObj) {
                 UnitsOnScreen.RemoveAt (i);
                 UnitObj.GetComponent<Unit> ().OnScreen = false;
                 return;
             }
 
             return;
         }
     }
     #endregion    
 }