Call to a member function getRelationExistenceQuery() on null

SelectConstraint::make('parent.metadata.title')
                            ->options(fn() => Asset::whereHas('parent')->with('parent.metadata')->get()->pluck('parent.metadata.title', 'parent.metadata.title'))
                            ->multiple()
                            ->nullable()
                            ->label('Belongs to')
                            ->relationship('parent.metadata', 'title'),


public function parent(): MorphTo
    {
        return $this->morphTo('parent');
    }