function sampGetListboxItemByText(text, plain)
     if not sampIsDialogActive() then return -1 end
        plain = not (plain == false)
     for i = 0, sampGetListboxItemsCount() - 1 do
         if sampGetListboxItemText(i):find(text, 1, plain) then
             return i
         end
     end
     return -1
end

attempt to call global 'sampGetListboxItemsCount' (a nil value)