very small fix for tabs: check for null content

[danice]

Proposed changes

This check is done in other places. Avoids problems when using tabs without automatic content selection.

Screenshots (if appropriate) or codepen:

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My commit messages follows the conventional commit format
• My change requires a change to the documentation, and updated it accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[wuda-io]

LGTM! The variable name _content could be improved in future. What does it store?
Looks more like an Element to me.