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