clear session fix.

Author: jchanvfx

NodeGraphQt/base/graph.py

@@ -692,7 +692,7 @@ def clear_session(self):
         Clears the current node graph session.
         """
         for n in self.all_nodes():
-            self.delete_node(n)
+            self._undo_stack.push(NodeRemovedCmd(self, n))
         self._undo_stack.clear()
         self._model.session = None