Laravel backpack select2_from_ajax setting my value as null after the correct value has been saved - laravel

Question

Im having a weird problem.
Im using laravel backpack for an admin panel. There i use select2_from_ajax to list a values according to another field in create operation. It is showing up correctly as expected & i can select one too.
But after selection when i click save & back it gives me an error
That means my column doesn't allow to update to null right.
So when i go back & check the column it has saved the correct value.
But when default value of my column was null this error will not showup & db value would be changed to null.
This is my select2_from_ajax part.
$this->crud->addField([ // Select
'label' => "Link Type",
'type' => 'select_from_array',
'name' => 'link_type', // the db column for the foreign key
'options' => [1 => 'Product',0 => 'Collection'],
'allows_null' => false,
]);
$this->crud->addField([ // Select
'label' => "Link To", // Table column heading
'type' => "select2_from_ajax",
'name' => "link_to",
'entity' => 'link',
'attribute' => "name",
'data_source' => url('admin/itemtype'),
'placeholder' => "Select a item",
'minimum_input_length' => 0,
'include_all_form_fields' => true,
'dependencies' => ['link_type'],
]);
So why is it trying to set null value after the correct value?
Any help would be appreciated. Thanks.
My admin/itemtype function:
$search_term = $request->input('q');
$form = collect($request->input('form'))->pluck('value', 'name');
if ($search_term) {
if ($form['link_type'] == 0) {
$items = Collection::where('name', 'LIKE', '%' . $search_term . '%')->paginate(10);
} else {
$items = Product::where('title', 'LIKE', '%' . $search_term . '%')->paginate(10);
}
} else {
if ($form['link_type'] == 0) {
$items = Collection::paginate(10);
} else {
$items = Product::paginate(10);
}
}
return $items;