Need Help with this movement Script

So far I'm been trying to figure out

1: why it's switching back and forth from grounded to air AFTER jumping and landing.

2: why it's keep on giving me a Null Reference Exception at lines 66 and 110.

3: how to keep the character grounded when walking down slopes

[RequireComponent (typeof (CharacterController))]

public class Movement : MonoBehaviour {

 /// This script moves the character controller forward 
 /// and sideways based on the arrow keys.
 /// It also jumps when pressing space.
 /// Make sure to attach a character controller to the same game object.
 /// It is recommended that you make only one call to Move or SimpleMove per frame.    
 
 
 
 public float speed = 6.0f;
 
 public float jumpHeight = 1.0f;
 public float gravity = 30.0f;
 public float maxFallSpeed = 20f;
     
 public float rotationSpeed = 15.0f;
 public float airSpeed  = 2.0f;
 public bool airControl = true;
 public bool canTurn = true;
 public bool canAirTurn = false;
 public bool canAirJump = false;
 public float maxAirJumps = 0f;    
 
 
 // Checking Movement states
 //bool isMoving = false;
 [SerializeField]
 private bool grounded = false;
 [SerializeField]
 private bool inAir = true;
 [SerializeField]
 private bool jumping = false;
 [SerializeField]
 private bool falling = true;
 private int airJumps = 0;

 // Private vars for stuff
 // Getting direction input
 private float xMove;
 private float zMove;
 
 private CharacterController controller;
 private Animator animator;
 private CollisionFlags collisions;
 private Transform mainCamPos;
 private float savedOffset;
 
 private Vector3 moveDirection = Vector3.zero;

// private Vector3 airVelocity = Vector3.zero; private float verticalSpeed;

 public bool debug = true;
 
 // Use this for initialization
 void Start () {
     controller  = GetComponent<CharacterController>();
     animator    = GetComponent<Animator>();
     mainCamPos  = Camera.main.transform;
     savedOffset = controller.stepOffset;        
     
     if(animator == null)
         return;
 }
 
 // Update is called once per frame
 void Update () {
     
     MovementLogic();
 
 }
 
 void FixedUpdate() {
     
     
     
 }
 
 void MovementLogic() {
 
     //Getting direction input
     xMove = Input.GetAxis("Horizontal");
     zMove = Input.GetAxis("Vertical");
     
    
     // Camera forward
     Vector3 forward = mainCamPos.forward;
     forward.y = 0;
     forward = forward.normalized;
     
     // Camera right
     Vector3 right = new Vector3(forward.z, 0, -forward.x);
         
     // Target direction relative to the camera
     moveDirection = xMove * right + zMove * forward;
     
     // Normalize when going diagnol on keyboard
      if (moveDirection.magnitude > 1.0f)
     {
         moveDirection = moveDirection.normalized;
     }
     
     // When the character is grounded on the floor
     if (controller.isGrounded) {    
         grounded = true;
         inAir = false;
         jumping = false;
         falling = false;
     }
     else {
         grounded = false;
         inAir = true; 
     }
     // When the character is falling in the air
     if (inAir && verticalSpeed < 0f){
         falling = true;
     }
     
     if(animator)
     {
         animator.SetFloat("speed", moveDirection.magnitude);
         animator.SetBool ("jumping", jumping);
         animator.SetBool ("falling", falling);
     }
     
     float pushDown = Mathf.Max (controller.stepOffset, new Vector3(moveDirection.x,0,moveDirection.z).magnitude);
     
     // While the character is grounded
     if (grounded){
         
         moveDirection -= pushDown * Vector3.up;    
         verticalSpeed = moveDirection.y;
         
         controller.stepOffset = savedOffset;
         airJumps = 0;
         
         Jump();
         
         //Moves appropriately with curves and hills

// verticalSpeed = Mathf.Min (verticalSpeed, 0);

     }
     
     // while the character is in air
     if (inAir){
         
         Gravitation();
         
         //To prevent sticking to the ceiling while moving in the air
         controller.stepOffset = 0.0f;
         
         // When your head hits the ceiling, you velocity flips down. Character Controller offset set
         // to zero to prevent sticking to ceiling.
         if((controller.collisionFlags & CollisionFlags.Above) > 0){
             verticalSpeed = -verticalSpeed;
         
         }
         
         //If you can air jump
         if (canAirJump && (airJumps < maxAirJumps)){
             Jump();
             airJumps++;
            }     
     }
     
     if (grounded || canAirTurn){
         // We are grounded, so recalculate
         // move direction directly from axes
         if (canTurn) {
             if(moveDirection != Vector3.zero) {
                 transform.rotation = Quaternion.Lerp(transform.rotation, Quaternion.LookRotation( new Vector3 (moveDirection.x, 0f, moveDirection.z)), Time.deltaTime * rotationSpeed);
             }
         }
     }
     
     
         
     
     // Merge Movement, speed, and gravity;
     Vector3 movement = moveDirection * speed + new Vector3(0, verticalSpeed, 0);
     
     // Move the controller
     controller.Move(movement * Time.deltaTime);
     
     // Debugging Code
     if (debug) {
         Debug.Log (verticalSpeed);
     
     }
 }
 

 void Jump() {
     if (Input.GetButtonDown ("Jump")) {
         
         jumping = true;
         verticalSpeed = CalculateJumpSpeed(jumpHeight);
     }
 }
 
 float CalculateJumpSpeed(float targetJumpHeight)
 {
     return Mathf.Sqrt (2 * targetJumpHeight * gravity);
 }
 
 //This is where the gravity comes from
 void Gravitation()
 {
     // Apply gravity and maximum fall speed
     verticalSpeed -= gravity * Time.deltaTime;
 }
 
 void SetVelocity(Vector3 newMovement)
 {
     moveDirection = newMovement;
 }

}