getUIElement(section_id) {
		const node = this.map.findElement('id', this.cbid(section_id));
		const inst = node ? dom.findClassInstance(node) : null;
		return (inst instanceof ui.AbstractElement) ? inst : null;
	}
Obtain the underlying `LuCI.ui` element instance.

@param {string} section_id
The configuration section ID

@throws {TypeError}
Throws a `TypeError` exception when no `section_id` was specified.

@return {LuCI.ui.AbstractElement|null}
Returns the `LuCI.ui` element instance or `null` in case the form
option implementation does not use `LuCI.ui` widgets.