Fly Mode 2D + Rigid Body vs CharacterController problems

Hello Guys !!

This is my very first question regarding unity scripting. I dont think i have understood the difference between a rigid boday and character controller :(

Im trying to make this kind of setup for my game : if the player presses the space bar on ground then he jumps (like duh) but when he presses it midair....instead of infinite jumping i want the player to enter a sort of "Fly Mode" where he can move in vertical and Horizontal ( a 2d plane) with WSAD.

flymode is just a boolean varible i made for the ability to use fly mode......

please help me out Main Error : NullReferenceException: Object reference not set to an instance of an object PlayerMovement.Start () (at Assets/Player/PlayerMovement.cs:27)

 NullReferenceException: Object reference not set to an instance of an object
 PlayerMovement.Start () (at Assets/Player/PlayerMovement.cs:71)

Also im not sure my algorithm for the fly mode is right

Thanks in advance !!!

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMovement : MonoBehaviour {
 
 
     Vector3 currentMovement;
     Vector3 targetMovement;
     Vector3 refMovement;
     
     public float moveSpeed = 10;
     public float flySpeed = 25;
     public float airControl = 0.8f;
     public float grndControl = 0.05f;
     private float moveSmoothener;
     
     //Gravity
     public float jumpSpeed = 500;
     bool flyMode = true;
     bool isFlying = false;
     
     CharacterController charControl;
 
     void Start () 
     {
         charControl = GetComponent<CharacterController> ();
         charControl.attachedRigidbody.useGravity = true;
     }
     
 
     void Update () 
     {
         if (Input.GetKeyDown (KeyCode.E))
             flyMode = !flyMode;
 
         targetMovement = new Vector3 (Input.GetAxisRaw ("Horizontal") * moveSpeed, 0, 0);
         if (isFlying)
             targetMovement = new Vector3 (Input.GetAxisRaw ("Horizontal") * flySpeed, Input.GetAxisRaw ("Vertical") * flySpeed, 0);
 
         currentMovement = Vector3.SmoothDamp (currentMovement, targetMovement, ref refMovement, moveSmoothener);
 
         if (charControl.isGrounded)
         {
             moveSmoothener = grndControl;
             isFlying = false;
             charControl.attachedRigidbody.useGravity = true;
         }
         if(Input.GetButtonDown("Jump"))
         {
             if(isFlying)
             {
                 charControl.attachedRigidbody.useGravity = true;
                 isFlying = false;
             }
             if (charControl.isGrounded)
             {
                 charControl.attachedRigidbody.AddForce(0,jumpSpeed,0);
                 moveSmoothener = airControl;
             }
             else if(charControl.isGrounded && flyMode)
             {
                 isFlying = true;
                 charControl.attachedRigidbody.useGravity = false;
             }
 
 
         }
 
 
 
         charControl.attachedRigidbody.MovePosition(transform.position + Time.deltaTime * currentMovement);
     
     }
 
 }