你好，我正在尝试设置GTA服务器，但遇到了以下错误

[    script:mrp-gangs] 脚本错误：@mrp-gangs/server.lua:822：尝试索引空值（本地变量'xKiller'）

[ script:mrp-gangs] > 处理程序 (@ReaperAC/server.lua:5)

RegisterServerEvent('asd:onPlayerDeath')
AddEventHandler('asd:onPlayerDeath', function(data)
    if data.killedByPlayer then
        local xKiller = ASD.GetPlayerFromId(data.killerServerId)
        TriggerClientEvent('asd_gangs:playerKilled', data.killerServerId)
    end
end)

RegisterServerEvent('asd_gangs:sumPlayerKill')
AddEventHandler('asd_gangs:sumPlayerKill', function(id)
    local xKiller = ASD.GetPlayerFromId(id)
    MySQL.Sync.execute("UPDATE users SET kills = kills + 1 WHERE identifier = @identifier", {['@identifier'] = xKiller.getIdentifier()})
end)