NullReferenceException when accessing Dependent Property of decomposed list element

[ssrmm]

In the below example Foo is a mandatory element of the Attribute list. However in the test program it is not present. In the current implementation this causes Foo to be null. Consequently setting or retrieving a property on it results in a NullReferenceException. Instead of null, Foo should be AmbiguousOrMissing.

---

Test Program:

Bar;

Grammar and AST:

namespace Test
{
  /***  Syntax  ***/

  syntax module TestSyntax
  {    
    using Nitra.Core;

    [StartRule]
    syntax Test = (Attribute ';' nl)+;

    token Attribute
    {
      | Foo = "Foo";
      | Bar = "Bar";
    }
  }

  /***  AST  ***/

  ast TestAst
  {
    Foo.Test = "Foo"; // System.NullReferenceException

    decompose Attributes
    {
      Foo : Attribute.Foo;
      Bar : Attribute.Bar;
    }

    Attributes : Attribute*;
  }

  abstract ast Attribute
  {
    | Foo { in Test : string; }
    | Bar {}
  }

  /***  Mapping  ***/

  map syntax TestSyntax.Test -> TestAst
  {
    Attributes -> Attributes;
  }

  map syntax TestSyntax.Attribute -> Attribute
  {
    | Foo -> Foo {}
    | Bar -> Bar {}
  }
}

[ssrmm]

The only workaround I've found so far is writing an extension method which does the null-checks and calling that method on the objet that is potentially null. However that's really confusing to read. Does anybody have a better idea?