Hey, how goes it.
If you check out the array list proxy, you'll see that its not adding the game objects like it is in the video. This is because if you look for the state "Add to List" it has Floats selected for the Type. We are not trying to place floats, we are trying to place the game object buttons that were selected in the state before.
Change the type from float to gameobject and set the variable to buttonObj _go
It might have been an error on my part or a conversion error from Unity 4 to Unity 5, either way, I got to the solution by investigating the array's value since the error came from there.