Select::make('product_id')
    ->relationship('product', 'title')
    ->placeholder('No products available')

    // This line is the fix
    ->fillEditOptionActionFormUsing(fn(string $state) => Product::find($state)?->toArray() ?? [])
    ->createOptionForm(fn(Form $form) => ProductResource::form($form)->operation('create') ?? [])
    ->editOptionForm(fn(Form $form) => ProductResource::form($form)->operation('edit') ?? [])
    ->manageOptionActions(fn(Action $action) => $action->modalWidth('6xl'))
    ->columnSpan(2)
    ->required()

Filament\Forms\Components\Select::getEditOptionActionFormData(): Return value must be of type array, null returned (View: /Users/helge/code/crescat/vendor/filament/forms/resources/views/components/select.blade.php)