Error: Object reference not set to an instance of an object

Hey im trying to make the player moves to where i click in the map and its showing me this error at line 20

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.AI;
5.  
6. public class PlayerMovement : MonoBehaviour {
7.     NavMeshAgent agent;
8.  
9.     void Start()
10.     {
11.         agent = GetComponent<NavMeshAgent>();
12.     }
13.  
14.     void Update()
15.     {
16.         if (Input.GetMouseButtonDown(0))
17.         {
18.             RaycastHit hit;
19.  
20.             if (Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit, 100))
21.             {
22.                 agent.destination = hit.point;
23.             }
24.         }
25.  
26.     }
27. }

 

Make dure you have a camera in your scene that has a MainCamera tag on it, else Camera.main won't find anything.