Ensure that the `viewer` property, on `BaseViewer`-instances, is a valid div-element (issue 12320)
Ensure that the `viewer` property, on `BaseViewer`-instances, is a valid div-element (issue 12320)

This should help prevent future issues, caused by the user omitting the `viewer` option and/or providing an incorrect `container` option, when initializing a `BaseViewer`-instance.

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