title: PySide2 not closing correctly with basic example
body: <p>When I run the basic script:</p>
  
  <pre><code>import sys
  from PySide2.QtWidgets import QApplication, QLabel
  
  app = QApplication(sys.argv)
  label = QLabel("Hello World")
  label.show()
  app.exec_()
  </code></pre>
  
  <p>forthe first time it all works fine. However, if I run it a second time I get:</p>
  
  <pre><code>File "../script.py", line 17, in &lt;module&gt;
  app = QApplication(sys.argv)
  
  RuntimeError: Please destroy the QApplication singleton before creating a new QApplication instance.
  </code></pre>
  
  <p>I am running the scripts in an Ubuntu machine. I get the same error in python2 and python3.</p>
  
  <p>Thanks !</p>