sorry.. only a short test.. the answer will follow

Qt Code: 
  1. //mainwindow.h
  2. //[SKIP]
  3. class MainWindow : public QMainWindow
  4. {
  5.     Q_OBJECT
  6.  
  7. public:
  8.     MainWindow();
  9.  
  10. protected:
  11.     void closeEvent(QCloseEvent *event);
  12.  
  13. private slots:
  14.    // [SKIP]
  15.     void about();
  16.     void indexEnv();  // first steps
  17.     void documentWasModified();
  18.  
  19. private:
  20.      void createActions();
  21.     //[SKIP]
  22.  
  23.     PreviewWindow *previewWindow; // first call of a sub dialog
  24.  
  25.     QTextEdit *textEdit;
  26.     QString curFile;
  27.  
  28.     QMenu *fileMenu;
  29.     QMenu *editMenu;
  30.     QMenu *indexMenu; // first steps to create an index menu --> my own menu
  31.     QMenu *helpMenu;
  32.  
  33.  
  34.     QToolBar *fileToolBar;
  35.     QToolBar *editToolBar;
  36.     //[SKIP]
  37.     QAction *aboutQtAct;
  38.     QAction *indexEnvAct; // first steps to create an index menu --> Action for my own menu
  39. };
  40.  
  41. // mainwindow.cpp
  42. //[SKIP]
  43. void MainWindow::createActions()
  44. {
  45.     //[SKIP]
  46.     aboutQtAct = new QAction(tr("About &Qt"), this);
  47.     aboutQtAct->setStatusTip(tr("Show the Qt library's About box"));
  48.     connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt()));
  49.  
  50.     indexEnvAct = new QAction(tr("&Index Environment settings"), this); // my own action and menu and so on ...
  51.     indexEnvAct->setStatusTip(tr("Opens the index environment setting dialog"));
  52.     connect(indexEnvAct, SIGNAL(triggered()), qApp, SLOT(indexEnv()));
  53.  
  54.     cutAct->setEnabled(false);
  55.     copyAct->setEnabled(false);
  56.     connect(textEdit, SIGNAL(copyAvailable(bool)),
  57.             cutAct, SLOT(setEnabled(bool)));
  58.     connect(textEdit, SIGNAL(copyAvailable(bool)),
  59.             copyAct, SLOT(setEnabled(bool)));
  60. }
  61.  
  62. void MainWindow::createMenus()
  63. {
  64. //[SKIP]
  65.  
  66.     menuBar()->addSeparator();
  67.  
  68.     indexMenu = menuBar()->addMenu(tr("&Index"));
  69.     indexMenu->addAction(indexEnvAct);
  70. }
  71.  
  72. void MainWindow::indexEnv()  // this here is the last part...
  73. {	
  74. 	previewWindow = new PreviewWindow(this);
  75. 	previewWindow->activateWindow();	
  76. 	previewWindow->show();
  77. }
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so, now I'm hopping, there is somewhere an error... which I can't find...
Qt Code: 
  1. // previewwindow.h
  2. #ifndef PREVIEWWINDOW_H
  3. #define PREVIEWWINDOW_H
  4.  
  5. #include <QWidget>
  6.  
  8. class QTextEdit;
  9.  
  10. class PreviewWindow : public QWidget
  11. {
  12.     Q_OBJECT
  13.  
  14. public:
  15.     PreviewWindow(QWidget *parent = 0);
  16.  
  17.   //  void setWindowFlags(Qt::WindowFlags flags);
  18.  
  19. private:
  20.     QTextEdit *textEdit;
  21.     QPushButton *closeButton;
  22. };
  23.  
  24. #endif
  25.  
  26. // previewwindow.cpp
  27.  
  28. #include <QtGui>
  29.  
  30. #include "previewwindow.h"
  31.  
  32. PreviewWindow::PreviewWindow(QWidget *parent)
  33.     : QWidget(parent)
  34. {
  35.     textEdit = new QTextEdit;
  36.     textEdit->setReadOnly(true);
  37.     textEdit->setLineWrapMode(QTextEdit::NoWrap);
  38.  
  39.     closeButton = new QPushButton(tr("&Close"));
  40.     connect(closeButton, SIGNAL(clicked()), this, SLOT(close()));
  41.  
  42.     QVBoxLayout *layout = new QVBoxLayout;
  43.     layout->addWidget(textEdit);
  44.     layout->addWidget(closeButton);
  45.     setLayout(layout);
  46.  
  47.     setWindowTitle(tr("Preview"));
  48. }
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Is there an error??
For me it seems correct... the 5 steps you described above seems for me to realised..

thanks in advanced