How to add item to my inventory when I pick them up?

I'm somewhat new to Unity and game development but I keep getting t$$anonymous$$s error:

 NullReferenceException: Object reference not set to an instance of an object
 HUD.InventoryScript_ItemAdded (System.Object sender, .InventoryEventArgs e) (at Assets/HUD.cs:21)
 inventory.AddItem (IInventoryItem item) (at Assets/Scripts/inventory.cs:42)
 collider.OnCollisionEnter (UnityEngine.Collision col) (at Assets/Scripts/collider.cs:38)

Usually when I get a NullReferenceException, the fixes is dragging in the object that a script is referring to. From what I can see here, the collider (w$$anonymous$$ch is on the player) is detecting the collision between the player and the item. It sent the information of collision object to the inventory class w$$anonymous$$ch adds the object to the List of inventory items. From here t$$anonymous$$ngs get a bit murky as it throws the error in the HUD.InventoryScript_ItemAdded method.

Here is the HUD.cs script

 using UnityEngine;
 using UnityEngine.UI;

 public class HUD : MonoBehaviour {

     public inventory Inventory;


     // Use t$$anonymous$$s for initialization
     void Start () {

         Inventory.ItemAdded += InventoryScript_ItemAdded;
     
     }

     private void InventoryScript_ItemAdded(object sender, InventoryEventArgs e)
     {
         Transform inventoryPanel = transform.Find("InventoryPanel");
         foreach(Transform slot in inventoryPanel)
         {

             //Border to image
             Image image = slot.GetC$$anonymous$$ld(0).GetC$$anonymous$$ld(0).GetComponent<Image>();

             //We found the empty slot
             if (!image.enabled)
             {
                 image.enabled = true;
                 image.sprite = e.Item.Image;

                 //TODO: Store reference to the item

                 break; //what does t$$anonymous$$s do?
         }
     }
  }
 

 

 // Update is called once per frame
 void Update () {
     
 }
 }  

T$$anonymous$$s is the inventory script

 public class inventory : MonoBehaviour {

 //fixed number of slots
 private const int SLOTS = 10;

 //creates an arraylist of inventory items
 private List<IInventoryItem> mItems = new List<IInventoryItem>();

 //handles event?
 public event EventHandler<InventoryEventArgs> ItemAdded;

 //adding item method
 public void AddItem(IInventoryItem item)
 {

     //if there are free slots available
     if (mItems.Count < SLOTS)
     {

         //collide with item
         Collider collider = (item as MonoBehaviour).GetComponent<Collider>();

         
         if (collider.enabled)
         {
             //if the collider of the object is enabled turn it off
             collider.enabled = false;

             //add to array list
             mItems.Add(item);

             item.OnPickup();

             //all subscribers of event are notfified?
             if(ItemAdded != null)
             {
                 ItemAdded(t$$anonymous$$s,new InventoryEventArgs(item));
             }
         }
     }
 }
 }

And finally t$$anonymous$$s is the collider script I put on my player

 public class collider : MonoBehaviour
 {
     public inventory Inventory;
     //public GameObject pickupEffect;
     public gun g;

     void OnTriggerEnter(Collider other)
 {
     
     if(other.gameObject.tag == "item")
     {
         Debug.Log("You Hit an item");
         Destroy(other.gameObject);

     }

     if(other.gameObject.tag == "ammo")
     {
         Debug.Log("Picked up ammo");
         Destroy(other.gameObject);
         g.ammo += 30;
     }
 }

 void OnCollisionEnter(Collision col)
 {
     
     if(col.gameObject.tag == "collectable")
     {
         Debug.Log("T$$anonymous$$s object is collectable");
         IInventoryItem item = col.gameObject.GetComponent<IInventoryItem>();
         if (item != null)
         {
             Inventory.AddItem(item);
             Debug.Log("Collided with an inventory item");
         }
     }

    
 }

 /*
  * 
 //check if item collided with has an inventory item interface
 private void OnControllerColliderHit(ControllerColliderHit $$anonymous$$t)
 {
     Debug.Log("Collided with an inventory item");
     IInventoryItem item = $$anonymous$$t.collider.GetComponent<IInventoryItem>();
     if(item != null)
     {
         Inventory.AddItem(item);

         Debug.Log("Collided with an inventory item");
     }
 }

 */

}

Sorry if the code is not formatted correctly but the system for creating code in t$$anonymous$$s forum seems to be a bit tedious because you have to space 4 times for each line.

Anyway, why am I getting t$$anonymous$$s error and how could I fix it w$$anonymous$$le implementing t$$anonymous$$s code?