QT Text Browser C++ Error

#include "firstqt.h"

FirstQt::FirstQt(QWidget *parent)

  : QMainWindow(parent)

{

  ui.setupUi(this);

  connect(ui.ExitBtn, &QPushButton::clicked, [this]  {  exitWindow();  } );

  connect(ui.UpdateBtn, &QPushButton::clicked, [this] {  updateTime();  } );

  state = 0;

}

FirstQt::~FirstQt()

{

}

void FirstQt::exitWindow()

{

  close();

}

void FirstQt::updateTime()

{

  ui.textBrowser.setText("Hallo");

}