Get items inside shulker box while it is in a GUI

I have a GUI where players can input any item, including shulker boxes. I want to make a procedure which reads what items are in the shulker box. Currently, I have tried logging both of these to the chat. As a test, I put a shulker box with 64 cobblestone in all of its slots, and the first procedure block said [Air] while the second was an empty message. I know that the inventory of the shulker box is located in tag:{BlockEntityTag:[{Slot:0,id:cobblestone,Count:64}]} but I don't know how to read data from it from a procedure. How can I achieve this?