title: PySide2 QUiLoader returns an empty window
body: <p>PySide2(5.6.0~a1) Qt UI file loader returns an empty window whereare PyQt5 loader works fine. Could you explained to me where I am wrong. </p>
  
  <p>Non Working PySide2 version:</p>
  
  <pre><code>import sys
  from PySide2.QtWidgets import QDialog, QApplication
  from PySide2 import QtUiTools
  
  class AppWindow(QDialog):
      def __init__(self):
          super().__init__()
          self.ui = QtUiTools.QUiLoader().load("dialog1.ui")
          self.show()
  
  app = QApplication(sys.argv)
  w = AppWindow()
  sys.exit(app.exec_())
  </code></pre>
  
  <p>Working PyQt5 version:</p>
  
  <pre><code>import sys
  from PyQt5.QtWidgets import QDialog, QApplication
  from PyQt5 import uic
  
  class AppWindow(QDialog):
      def __init__(self):
          super().__init__()
          self.ui = uic.loadUi("dialog1.ui", self)
          self.show()
  
  app = QApplication(sys.argv)
  w = AppWindow()
  sys.exit(app.exec_())
  </code></pre>
  
  <p>Using this function also does not work :</p>
  
  <pre><code>def loadUiWidget(uifilename, parent=None):
      loader = QtUiTools.QUiLoader()
      uifile = QtCore.QFile(uifilename)
      uifile.open(QtCore.QFile.ReadOnly)
      ui = loader.load(uifile, parent)
      uifile.close()
      return ui
  </code></pre>