BackdropNode _deserialize bug and fix

[Soooya]

When I open a save file containing BackDropNode, following error msg are shown:

Traceback (most recent call last): File ".\Python310\lib\site-packages\nodegraphqt-0.2.2-py3.10.egg\NodeGraphQt\base\graph_actions.py", line 119, in _open_session graph.load_session(file_path) File ".\Python310\lib\site-packages\nodegraphqt-0.2.2-py3.10.egg\NodeGraphQt\base\graph.py", line 1429, in load_session self.import_session(file_path) File ".\Python310\lib\site-packages\nodegraphqt-0.2.2-py3.10.egg\NodeGraphQt\base\graph.py", line 1452, in import_session self._deserialize(layout_data) File ".\Python310\lib\site-packages\nodegraphqt-0.2.2-py3.10.egg\NodeGraphQt\base\graph.py", line 1314, in _deserialize if prop in node.view.widgets: AttributeError: 'BackdropNodeItem' object has no attribute 'widgets'

A temporary fix: add self.widgets = {} in BackdropNodeItem.__init__()