Lots of bugs that I don't know what might be causing them

Apparently my procedures are right, but many bugs start to occur.

I did that when you clicking the "prepare" button opens the inventory GUI I created and then puts the item in the output slot. However when I click the button again the item I took earlier disappears and another one appears in the exit slot. Also when I click the button
"prepare" and exit the current GUI and enter the new inventory by pressing the "o" key the item is no longer there and when I drop the item it also disappears. Can someone help me please.

Video: https://youtu.be/s9e5P4r0v2E