myPS->show();
myPS->raise();
myPS->activateWindow();

}@

If i do not use the if-condition i can open as much forms as i want. Anyway i want to do this check,
but do not know where the error is.
mainwindow.h:
@#include <QMainWindow>
#include "portsetting.h"

#include <QDebug>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private slots:
void on_pb_test_1_clicked();

private:
Ui::MainWindow *ui;
PortSetting *myPS;

};@

portsetting.h :
@#include <QMainWindow>
#include <QDialog>

namespace Ui {
class PortSetting;
}

class PortSetting : public QDialog
{
Q_OBJECT

public:
explicit PortSetting(QWidget *parent = 0);
~PortSetting();

private:
Ui::PortSetting *ui;

};
@

portsetting.cpp:
@#include "portsetting.h"
#include "ui_portsetting.h"

PortSetting::PortSetting(QWidget *parent) :
QDialog(parent),
ui(new Ui::PortSetting)
{
ui->setupUi(this);
}

PortSetting::~PortSetting()
{
delete ui;
}@

there is portsetting.ui but i think its not necessary to post here.
The form contains a single pushbutton connected to PortSetting close()