The variable GroundCheck has not been assigned

Here is the code:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class PlayerMovement : MonoBehaviour { public CharacterController controller;

 public float speed = 12f;
 public float gravity = -9.81f;
 public float jumpHeight = 3f;

 public Transform groundCheck;
 public float groundDistance = 0.4f;
 public LayerMask groundMask;

 Vector3 velocity;
 bool isGrounded;

 // Update is called once per frame
 void Update()
 {
     isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

     if (isGrounded && velocity.y < 0) 
     {
         velocity.y = -2f;
     }

     float x = Input.GetAxis("Horizontal");
     float z = Input.GetAxis("Vertical");

     Vector3 move = transform.right * x + transform.forward * z;

     controller.Move(move * speed * Time.deltaTime);

     if(Input.GetButtonDown("Jump") && isGrounded)
     {
         velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
     }

     velocity.y += gravity * Time.deltaTime;

     controller.Move(velocity * Time.deltaTime);
 }

}