The character controller return null refrence

I try to make my character move by using CharacterController but the problem is in the run time it tell me that the character controller does not match with any anything and the character do not move at all, so is there any falt in the code:

Here is my code:

 using UnityEngine;
 using System.Collections;
 
 public class Move {
     
 
 
     Transform transform;
     float gravity  = 9.8f;
     float  speed  = 3f;
      float jumpSpeed  = 0.3f;
     Animation animation;
     CharacterController controller = new CharacterController();
     Vector3 moveDirection = Vector3.zero;
 
     
      public    void Moving() {
         
         controller = (CharacterController)controller.GetComponent(typeof(CharacterController));
     
     
         transform.Rotate(0, 2 * Input.GetAxis("Mouse X"),0);
     
         if(controller.isGrounded){
     
             moveDirection.x = 0;
             moveDirection.z = 0;
         
     
             if(Input.GetKey("up")){
             
 
                 moveDirection = transform.TransformDirection(Vector3.forward);    
                 controller.Move(moveDirection * Time.deltaTime * speed);
                 animation.Play("walk");
             
             }
         
             if(Input.GetKey(KeyCode.Space)){
                 moveDirection = transform.TransformDirection(Vector3.forward);    
                 controller.Move(moveDirection * Time.deltaTime * speed);
                 animation.Play("run");
             }
         
             if(Input.GetKeyUp(KeyCode.Space)){
         
                 animation.Play("idle");
             }
         
             if(Input.GetKey("down")){
                 moveDirection = -transform.TransformDirection(Vector3.forward)* 2;    
                 controller.Move(moveDirection * Time.deltaTime * speed );
                 animation.Play("walk");
 
             }
         
         }
     
 
         moveDirection.y -= gravity*Time.deltaTime;
         controller.Move(moveDirection * Time.deltaTime);
 
         }
     }