fix: set ArcRendererElement<D> to ArcRendererElement<Object>

[Riley-zyx]

Fix error _CastError (type 'List<ArcRendererElement<dynamic>>' is not a subtype of type 'List<ArcRendererElement<Object>>?' in type cast)

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[stuartmorgan]

This project is no longer maintained, and cannot accept any PRs. Community members interested in collaborating on a community-maintained fork can coordinate in this issue.