When clicking the minimap button the errors just start scrolling non-stop.

It seems there's already a PR with a better fix.
fe39362

The GetMouseFocus function has been replaced by GetMouseFoci.

https://warcraft.wiki.gg/wiki/Patch_11.0.0/API_changes

I got it working by swapping
local frame = GetMouseFocus()
to
local frame = GetMouseFoci()[1]

Managed to copypaste the error from BugSack:

4442x ItemRack/ItemRack.lua:1289: attempt to call global 'GetMouseFocus' (a nil value)
[string "@ItemRack/ItemRack.lua"]:1289: in function `func'
[string "@ItemRack/ItemRack.lua"]:1016: in function `OnUpdate'
[string "*ItemRack.xml:7_OnUpdate"]:1: in function <[string "*ItemRack.xml:7_OnUpdate"]:1>

Locals:
(*temporary) = nil
(*temporary) = "attempt to call global 'GetMouseFocus' (a nil value)"

What's the plan here? This game is basically unplayable as a tank without this addon

Have a fix made #270

Can provide a zip for those who need it until it is merged

Released as 4.11