Help me to understand what is the problem with my script,pls.

I'm new at C# and I don't understand what is the problem with my scripts. Sorry for my English. Unity throw an NullReferenceException:
Object reference not set to an instance of an object
Tile.Priority (System.String playerObject) (at Assets/My Files/Scripts/Tile.cs:18)
UnityEngine.Component:BroadcastMessage(String, Object)
ComputerTrigger:Start() (at Assets/My Files/Scripts/ComputerTrigger.cs:12)
in these scripts:

Code (csharp):

1. using UnityEngine;
2. using System.Collections;
3. using System.Collections.Generic;
4.  
5. public class Tile : MonoBehaviour {
6.  
7. [INDENT]   public Dictionary <string, int> priorityMap;[/INDENT]
8.     private Collider[] coll;
9.     private string playerObject;
10.     private int priority = 0;
11.     private static Dictionary <string, int> tmpMap;
12.  
13.     //Debug
14.     public int Check;
15.  
16.     public void Priority(string playerObject){
17.         priorityMap.Add (playerObject,0);
18.         this.playerObject = playerObject;
19.         for (int i = 0; i<4; i++) {
20.             CheckTilesAround(i);
21.         }
22.     }
23.     void getMap(){
24.         tmpMap = priorityMap;
25.     }
26.  
27.     void CheckTilesAround(int arg){
28.         //0 - left, 1 - up, 2 - right, 3 - down (relative global coordinates)
29.         lock (this) {
30.             if (arg == 0) {
31.                 coll = Physics.OverlapSphere (new Vector3 (gameObject.transform.position.x - 1f, gameObject.transform.position.y, gameObject.transform.position.z), 0.5f);
32.                 if (coll != null) {
33.                     for (int i = 0; i < coll.Length; i++) {
34.                         if (coll .name == "Tile") {
35.                             CheckPriority();
36.                             getMap();
37.                             coll.GetComponent<Tile>().BroadcastMessage("Priority",playerObject);
38.                         }
39.                     }
40.                 }
41.             } else if (arg == 1) {
42.                 coll = Physics.OverlapSphere (new Vector3 (gameObject.transform.position.x, gameObject.transform.position.y, gameObject.transform.position.z + 1f), 0.5f);
43.                 if (coll != null) {
44.                     for (int i = 0; i < coll.Length; i++) {
45.                         if (coll .name == "Tile") {
46.                             CheckPriority();
47.                             getMap();
48.                             coll.GetComponent<Tile>().BroadcastMessage("Priority",playerObject);
49.                         }
50.                     }
51.                 }
52.             } else if (arg == 2) {
53.                 coll = Physics.OverlapSphere (new Vector3 (gameObject.transform.position.x + 1f, gameObject.transform.position.y, gameObject.transform.position.z), 0.5f);
54.                 if (coll != null) {
55.                     for (int i = 0; i < coll.Length; i++) {
56.                         if (coll .name == "Tile") {
57.                             CheckPriority();
58.                             getMap();
59.                             coll.GetComponent<Tile>().BroadcastMessage("Priority",playerObject);
60.                         }
61.                     }
62.                 }
63.             } else if (arg == 3) {
64.                 coll = Physics.OverlapSphere (new Vector3 (gameObject.transform.position.x, gameObject.transform.position.y, gameObject.transform.position.z - 1f), 0.5f);
65.                 if (coll != null) {
66.                     for (int i = 0; i < coll.Length; i++) {
67.                         if (coll .name == "Tile") {
68.                             CheckPriority();
69.                             getMap();
70.                             coll.GetComponent<Tile>().BroadcastMessage("Priority",playerObject);
71.                         }
72.                     }
73.                 }
74.             }
75.         }
76.     }
77.     void CheckPriority(){
78.         if (tmpMap != null) {
79.             Dictionary<string,int>.Enumerator enumeratorTmp = tmpMap.GetEnumerator ();
80.             while (enumeratorTmp.MoveNext()) {
81.                 KeyValuePair<string,int> pairTmp = enumeratorTmp.Current;
82.                 if (pairTmp.Key.Equals (playerObject))
83.                     priority = pairTmp.Value + 1;
84.             }
85.         }
86.         if (priorityMap.ContainsKey (playerObject)) {
87.             Dictionary<string,int>.Enumerator enumerator = priorityMap.GetEnumerator ();
88.             while (enumerator.MoveNext()) {
89.                 KeyValuePair<string,int> pair = enumerator.Current;
90.                 if (pair.Key.Equals (playerObject) && priority < pair.Value) {
91.                     priorityMap.Remove (playerObject);
92.                     priorityMap.Add (playerObject, priority);
93.                     Check = priority;
94.                 }
95.             }
96.         } else {
97.             priorityMap.Add (playerObject, priority);
98.             Check = priority;
99.         }
100.     }
101. }

and

Code (csharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class ComputerTrigger : MonoBehaviour {
5.  
6.     private Collider[] tile;
7.  
8.     void Start () {
9.         tile = Physics.OverlapSphere (new Vector3(gameObject.transform.position.x,gameObject.transform.position.y - 0.5f,gameObject.transform.position.z), 1f);
10.         if(tile[0].name == "Tile"){
11.             tile[0].GetComponent<Tile>().BroadcastMessage("Priority","Computer");
12.  
13.         }
14.     }
15.  
16. }

Please explain me what is the problem.

P.s. Method BrodcastMessage create a new thread or continue current one?

 

Code tags please.

 

Change line 7 to

Code (csharp):

1. public Dictionary <string, int> priorityMap = new Dictionary <string, int>();

In general it's normally better to write scripts from scratch when you start off, not copy and paste stuff iff the internet.

 

This is a good spot to start, they cover the basics & step you up to more complex stuff

https://unity3d.com/learn/tutorials

 

Thank you very much, I knew adout it, but forget