Call to a member function getRelationExistenceQuery() on null

I am using this filter:
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');
    }

the parent is a morphTo relation from Asset. The loading of options is executed normally. But then when I select an option I get the error 'Call to a member function getRelationExistenceQuery() on null'.

Parent morphs now to 2 different models. Strange thing is that if parent only uses (morphs to) 1 model there is NO error and everthing works correctly. PLEASE HELP!
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