Pull request overview

• Fixes No error for orphan ConnectorList object #11211

Description of the purpose of this PR

Add severe error when orphan Splitter, Mixer, or ConnectorList objects are found.
Add unit test for orphan Splitter, Mixer, ConnectorList, Branch, and BranchList.

Pull Request Author

• Title of PR should be user-synopsis style (clearly understandable in a standalone changelog context)
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Reviewer

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