Workaround about QSettings.value of recentFiles for PyQt4

labelme/app.py

@@ -418,7 +418,8 @@ class MainWindow(QtWidgets.QMainWindow, WindowMixin):
         # XXX: Could be completely declarative.
         # Restore application settings.
         self.settings = QtCore.QSettings('labelme', 'labelme')
-        self.recentFiles = self.settings.value('recentFiles', [])
+        # FIXME: QSettings.value can return None on PyQt4
+        self.recentFiles = self.settings.value('recentFiles', []) or []
         size = self.settings.value('window/size', QtCore.QSize(600, 500))
         position = self.settings.value('window/position', QtCore.QPoint(0, 0))
         self.resize(size)