Coming from https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/482145/1/resources/src/mediawiki.special.apisandbox/apisandbox.js OutlineOptionWidget nor any of its extended widgets seem to mixin TitledElement, therefore resulting in an error.
Widgets supposed to carry TitledElement mixin (already or proposed: ✓) or not (×):
- ButtonWidget ✓
- ActionWidget
- PopupButtonWidget
- ButtonGroupWidget ✓
- DropdownWidget ✓
HiddenInputWidget×- IconWidget ✓
- IndicatorWidget ✓
- InputWidget ✓
- ButtonInputWidget
- CheckboxInputWidget
- CheckboxMultiselectInputWidget
- DropdownInputWidget
- RadioInputWidget
- RadioSelectInputWidget
- TextInputWidget
- ComboboxInputWidget
- MultilineTextInputWidget
- NumberInputWidget
- SearchInputWidget
- LabelWidget ✓
- MultioptionWidget ✓
- CheckboxMultioptionWidget
- MultiselectWidget ✓ (?)
- CheckboxMultiselectWidget
- OptionWidget ✓
- ButtonOptionWidget
- DecoratedOptionWidget
- MenuOptionWidget
- MenuSectionOptionWidget
- OutlineOptionWidget
- RadioOptionWidget
- TabOptionWidget
- OutlineControlsWidget (? ButtonWidgets already carry title out of box)
PopupWidget×- ProgressBarWidget (?)
- SearchWidget (?)
- SelectFileWidget ✓
- SelectWidget ✓
- ButtonSelectWidget
- MenuSelectWidget
- OutlineSelectWidget
- RadioSelectWidget
- TabSelectWidget
- TagItemWidget (? ButtonWidget carries title out of box)
- TagMultiselectWidget ✓
- MenuTagMultiselectWidget
- PopupTagMultiselectWidget (deprecated)
- ToggleWidget ✓
- ToggleButtonWidget
- ToggleSwitchWidget