Qt nextPendingConnection（）返回NULL

Question

我有一个从QTcpServer继承的Server类。我已连接slot和s信号，但QTcpServer :: nextPendingConnection（）和Server :: nextPendingConnection（）返回NULL。我该怎么办？ 这是服务器的实现

Server::Server(QObject *parent) : QTcpServer(parent)
{
    tcpSocket = new QTcpSocket();
    start(QHostAddress::Any, quint16(port));
}

void Server::start(const QHostAddress &address, quint16 port)
{
    connect(this, &Server::newConnection, this, &Server::slotNewConnection);

    if (!this->listen(address, port))
    {
        ....
    }
    else
    {
        ....
    }
}
    void Server::slotNewConnection() 
{
    QTcpSocket *client = this->nextPendingConnection(); //RETURNS NULL
    clients.insert(client);
    client->write("Hello");
    connect(client, &QTcpSocket::readyRead, this,           &Server::slotServerRead);
    connect(client, &QTcpSocket::disconnected, this, &Server::slotClientDisconnected);
}

Server.h文件：

class Server : public QTcpServer
{
    Q_OBJECT
public:

    explicit Server(QObject *parent = 0);

    void start(const QHostAddress &address, quint16 port);

    void sendMessageToGroup();

public slots:

    void slotNewConnection();
    void slotServerRead();
    void slotClientDisconnected();

private:

    QSet<QTcpSocket*> clients;
};