NullReferenceException: Object reference not set to an instance of an object

Hello I am trying to make a game and keep getting this error message:

NullReferenceException: Object reference not set to an instance of an object

GUI_HUD.OnGUI () (at Assets/Scripts_/GUI_HUD.js:23).

And I for one have no idea what it means and my friend and I have trying to figure it out. The code of the GUI_HUD looks like this

var customSkin:GUISkin;

var EXPImage:Texture2D;

var healthpotionImage:Texture2D;

var manapotionImage:Texture2D;



private var customControls:GUI_CustomControls;

private var playerInvo:Player_Inventory;



function Awake()

{

	customControls = FindObjectOfType(GUI_CustomControls);

	playerInvo = FindObjectOfType(Player_Inventory);

}



function OnGUI()

{

	if(customSkin)

	{

		GUI.skin = customSkin;

	}

		

	//Inventory Buttons --------------------------------------

	if(customControls.InvoHudButton(Rect(10, Screen.height - 100, 93, 95), playerInvo.GetItemCount(InventoryItem.MANAPOTION), manapotionImage, "Click To Use a Mana Potion"))

	{

		playerInvo.UseItem(InventoryItem.MANAPOTION, 1);

	}

	

	if(customControls.InvoHudButton(Rect(110, Screen.height - 100, 93, 95), playerInvo.GetItemCount(InventoryItem.HEALTHPOTION),healthpotionImage, "Click To Use a Health Potion"))

	{

		playerInvo.UseItem(InventoryItem.HEALTHPOTION, 1);

	}

	

	//Non Usable Inventory Buttona ------------------------------

	customControls.InvoHudButton(Rect(Screen.width - 210, Screen.height - 100, 93, 95), playerInvo.GetItemCount(InventoryItem.EXP), EXPImage, "Amount of EXP");

}



function Start () {



}

the GUI_CustomControls code looks like this

function InvoHudButton(screenPos: Rect, numAvailable : int, itemImage: Texture, itemtooltip: String ) : boolean
{	
		if( GUI.Button(screenPos, GUIContent(itemImage, itemtooltip), "HUD Button") )
		{ 
			return true;
		}
		GUI.Label( Rect(screenPos.xMax - 20, screenPos.yMax - 25, 20, 20 ), numAvailable.ToString() );
		
		//display area for tooltips
		GUI.Label( Rect( 20, Screen.height - 130, 500, 100), GUI.tooltip);
}

have a look, the error is online 23, it says in the error message. It means that the command you wrote on line 23 is calling an object/component, except that you haven’t specified properly the componentthat it supposed to call, the variable is empty basically something to that effect. so you have to figure out a way to specify the variable that it is calling so is properly initialised to a game objects/components/it has found something to execute on.