Edit multiple checkbox value in blazor server side

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Blazor

Hey can you please tell me how to edit multiple checkbox values
for create i use

public void ChangeCheckbox(int id)
{
var feature = FeaturesList.FirstOrDefault(x => x.FeatureId == id);

product.FeaturesList.Add(feature);
}
this code while adding and its successfully run
but when i use to edit it
System.NullReferenceException: 'Object reference not set to an instance of an object.' CrudWith Radzen.ViewModel.Product ViewModel.Features List.get returned null. this type of error occur

What I have tried:

public void ChangeCheckbox(int id)
    {
        var feature = FeaturesList.FirstOrDefault(x => x.FeatureId == id);
       
        product.FeaturesList.Add(feature);
}