change custom field/system setting lookup to 'null' or empty

[pricerc]

If I've got a "lookup" cell type on a custom field or system setting, the value starts out as NULL/empty.

But once the value is set, it seems I am unable to find where to 'unset' it.

So what's the trick that I've missed?

(edit: I do realise that this is a minor problem for a system setting, because you can just go into the plugin maintenance screen and clear it there, but that doesn't work for a custom field)

[SBarnes]

Take a look at viewtopic.php?f=26&t=1103&p=4249&hilit=clear+grid#p4249 Scott provides an example