Setup empty Dock at startup.

def setupDock(self):
        """Setup empty Dock at startup. """
        self.dock = QtWidgets.QDockWidget("Classes", self)
        self.dock.setWidget(self.tree)
        self.dock.setFeatures(QtWidgets.QDockWidget.NoDockWidgetFeatures)
        self.addDockWidget(QtCore.Qt.LeftDockWidgetArea, self.dock)