I made a block with inventory. When trying my mod in runclient, I was able to use hoppers with that block.
So made a GUI and bound its GUI to that block. However, when I was adding a slot to that GUI it said that I need to add an inventory before adding slots.
So I thought that the bug was in the block and tried to bind that gui to another block with inventory that was made before 1.8.0 and I had the same issue.