title: QT QTextEdit setText crashes
body: <p>I've created a qt widgets application. Using the design mode I've created a QTextEdit and indicated that in the header file: </p>
  
  <pre><code>...
  
  QT_BEGIN_NAMESPACE
  class QAction;
  class QMenu;
  class QTextEdit;
  QT_END_NAMESPACE
  
  ...
  
  private:
      Ui::MainWindow *ui;
      QTextEdit *textEdit_2;
  };
  </code></pre>
  
  <p>There is also a slot which is triggered by pushing a button. What it has to do is to insert some text into textEdit_2 after the button is pushed, still the program crashes.
  In mainwindow.cpp:</p>
  
  <pre><code>void MainWindow::on_action_4_triggered()
  {
      textEdit_2-&gt;setText("text");
  }
  </code></pre>
  
  <p>I've also tried </p>
  
  <pre><code>textEdit_2-&gt;setText(QString("text"));
  </code></pre>
  
  <p>which anyway doesn't work. What's the problem?</p>