This PR fixes a problem with widgets not initializing properly when integrating with third-party libraries (such as ipywidgets) in v6.0.0: #4998

It seems that some of the changes in #4956 modified variable names used to load the widget's layout and data.

(削除) Let me know if adding test cases for this is required (I believe it'd be hard to test since it relies on other libraries). (削除ここまで)
Edit: Added basic tests just in case!

Thank you!

Before fix

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After fix

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