Fix regression in TreeWidget: original widget can be overridden

[penguinolog]

Use base_widget instead

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 8263613235

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.413%

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Totals
Change from base Build 8263035088:	0.0%
Covered Lines:	9291
Relevant Lines:	12627

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💛 - Coveralls