Improve clearing canvas.

main_window.py

@@ -169,6 +169,7 @@ class MainWindow(QMainWindow, Ui_MainWindow):
         self.opengl_widget.update()
 
     def _clear_canvas(self):
+        self._reset()
         PointManager.point_set.clear_points()
         refresh_point_list(self)
         self.opengl_widget.update()