I am sorry I found the mistake. As stated in the thread I checked the source, not the destination. Sorry!

 

 

For a composter and a hopper underneath InventoryMoveItemEvent#getSource#getHolder is not an instance of Hopper

private Location getHoppersLocation(InventoryHolder holder) {
if (holder instanceof Hopper)

{ Hopper hopper = (Hopper) holder; return hopper.getLocation().clone(); }

if (holder instanceof HopperMinecart)

{ HopperMinecart hopperMinecart = (HopperMinecart) holder; return hopperMinecart.getLocation().clone(); }

new DebugMessage(this.getClass(), plugin, "HOLDER-CLASS = " + holder.getClass().getName());
return null;
}

This method returns null for InventoryMoveItemEvent#getSource#getHolder for me. Output of the debug-message: HOLDER-CLASS = org.bukkit.craftbukkit.v1_14_R1.inventory.CraftBlockInventoryHolder

 

This is (at least for me) crucial because otherwise I have no other way to get the location and I believe this is a bug.