Immortal blocks after changing custom block slot types

[dragazo]

Reproduction steps:

• start with a custom block that has two slots
• create an instance of the custom block and put any block in the second slot
• edit the custom block and change the type of the first slot
• drag the contents of the second slot into the palette

The result is a block hovering at a fixed point in the palette (but only in that category) which cannot be deleted or dragged, but can otherwise be interacted with like a normal block in the scripts area (click to run, script pick, duplicate, etc.). Changing the slot type a second time seems to get rid of it.