title: QT GraphicsView become black
body: <p>I have this simple Dialog in QT:
  <img src="https://i.stack.imgur.com/ajDOV.png" alt="enter image description here"></p>
  
  <p>when I click on "opzioni" appears a new block of buttons..but the two white object (QGraphicsView) become black..
  I can't make a screenshot because when I press a key or move it, it becomes white again XD</p>
  
  <p>I tried overloading resizeEvent()</p>
  
  <pre><code>void Dialogo::resizeEvent ( QResizeEvent * e )
  {
   if(immagine)
   immagine-&gt;update();
   if(mappa)
   mappa-&gt;update();
  
   QDialog::resizeEvent(e);
  }
  </code></pre>
  
  <p>whith immagine and mappa my QGraphicsView
  but nothing. Why this ?</p>
  
  <p>In my Dialog constructor I have</p>
  
  <pre><code>   setupUi(this);
     layout()-&gt;setSizeConstraint(QLayout::SetFixedSize);
  </code></pre>
  
  <p>The final result when I press "opzioni" would be
  <img src="https://i.stack.imgur.com/OuNiJ.png" alt="enter image description here"></p>