title: QComboBox - Select no entry
body: <p>I have a QComboBox on my ui and set the model like this:</p>
  
  <pre><code>QStringListModel* model = new QStringListModel;
  QStringList stringlist;
  stringlist &lt;&lt; "Test1" &lt;&lt; "Test2" &lt;&lt; "Test3";
  
  model-&gt;setStringList(stringlist);
  ui-&gt;comboBox-&gt;setModel(model);
  </code></pre>
  
  <p>Now I want to change the current index to be none (so that I get a blank combo box).</p>
  
  <p>I already tried setting the current index to -1 with <code>ui-&gt;comboBox-&gt;setCurrentIndex(-1);</code>, but that results to an index aout of range exeption in qlist:</p>
  
  <pre><code>ASSERT failure in QList&lt;T&gt;::operator[]: "index out of range", file F:/Qt/5.4/mingw491_32/include/QtCore/qlist.h, line 486
  </code></pre>