NullReferenceException: Object reference not set to an instance of an object PlayerControl.Update () (at Assets/PlayerControl.cs:22)

Hi! I'm new to unity and i found an error when i click the Left mouse button, it's just give me the error NullReferenceException: Object reference not set to an instance of an object PlayerControl.Update () (at Assets/PlayerControl.cs:22)

PlayerControl.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 [RequireComponent(typeof(PlayerMotor))]
 public class PlayerControl : MonoBehaviour
 {
     PlayerMotor motor;
     public LayerMask movementMask;
     Camera cam;
     // Start is called before the first frame update
     void Start()
     {
         cam = Camera.main;
         motor = GetComponent<PlayerMotor>();
     }
 
     // Update is called once per frame
     void Update()
     {
         if(Input.GetMouseButtonDown(0))
         {
             Ray ray = cam.ScreenPointToRay(Input.mousePosition);
             RaycastHit hit;
             if(Physics.Raycast(ray, out hit, 100, movementMask))
             {
                 Debug.Log("We hit " + hit.collider.name + "" + hit.point);
                 motor.MoveToPoint(hit.point);
             }
         }
         
     }
 }

PlayerMotor.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.AI;
 [RequireComponent(typeof(NavMeshAgent))]
 public class PlayerMotor : MonoBehaviour
 {
     NavMeshAgent agent;
     // Start is called before the first frame update
     void Start()
     {
         agent = GetComponent<NavMeshAgent>();    
     }
 
     public void MoveToPoint(Vector3 point)
     {
         agent.SetDestination(point);
     }
 }

This is my first question in this forum, please help me!