I am sending information from my item to my inventory. Telling the inventory to add the item. Gives me a null reference exception.
this is not a GameObject; its class is derived from MonoBehaviour. You use the correct semantics later on:
Using the shorthand for this.gameObject works fine:
However, you should use something other than an ArrayList; a List is probably what you want. Using .gameObject on item is useless; I don't even know why that compiles.