Items I put into my GUI are being destroyed

I am trying to make a lunchbox that I can put items into, like a bundle but a shulker box, so it can hold 4 slots of anything. My first problem was the game crashing when I set the max slots to 4 when there were 4 in the gui, but I fixed that by setting it to 5. Now I have a problem where when I put some items in the slots (just random items, such as dark oak planks and aluminum swords), the items get destroyed when I open up the lunchbox again. Is this because I set it to not drop items when gui is not bound to any external inventory is closed? I just need a solution because my friends are wanting me to put my mod on my server asap