private function load()
    {
        if ($this->users) {
            return null;
        }
        $content = $this->io->load();
        $this->users = [];
        foreach (explode("\n", $content) as $line) {
            $line = trim($line);
            if ($line !== '') {
                $user = User::loadFromFileLine($line);
                if ($user === null) {
                    return $this->invalid();
                }
                $this->users[$user->getName()] = $user;
            }
        }
        return null;
    }