`__call__` of `SubgraphX` returns List instead of `MCTSNode`

[Michael1015198808]

It seems that the __call__ method of SubgraphX returns wrong type that made me thought List is the expected type.
In __call__, the explanation_results appends items from explain (line 855)
While in line 818 of explain, the item is changed into List by method write_from_MCTSNode_list.
Since the suspicious code is located into the lowest level of code, I hope this bug could be fixed.