error when clearing "First Button Slot" settings field

[jsb]

Reported by ardfz on wowinterface.com.

When I delete all numbers in the first button slot, I get this error:

Message: Interface\AddOns\RingMenu\main.lua:94: attempt to perform arithmetic on field 'firstSlot' (a nil value)
Time: Mon Aug 12 15:20:58 2019
Count: 2
Stack: Interface\AddOns\RingMenu\main.lua:94: attempt to perform arithmetic on field 'firstSlot' (a nil value)
[C]: ?
Interface\AddOns\RingMenu\main.lua:94: in function `RingMenu_UpdateRing'
Interface\AddOns\RingMenu\options.lua:96: in function <Interface\AddOns\RingMenu\options.lua:86>

Locals:

[jsb]

fixed in 576019c