Object reference not set to an instance of an Object?

Hi, I'm new to Unity, I've backed a Navmesh in my game and wanted to use mouse click to set destination for player, but I got this error in console so the agent doesn't move at all, can someone explain where went wrong? The error is in the line "if (Physics . Raycast (Camera .main. ScreenPointToRay (Input .mousePosition), out hit, 100)) ". Here my script:

 using UnityEngine;
 using System.Collections;
 
 public class Navmesh : MonoBehaviour {
      NavMeshAgent agent;
   
     void Start () {
         agent = GetComponent<NavMeshAgent>();
 
     }
     
     void Update () {
     if (Input.GetMouseButtonDown (0))
         {
             
             RaycastHit hit;
             if (Physics . Raycast (Camera .main. ScreenPointToRay (Input .mousePosition), out hit, 100))
             {
                 agent.destination =hit.point;
             }
         }
     }
 }