+= operator causing NullReferenceException when use on class property

[rismans]

Noticed this on the last two beta releases.

When using the += operator on a property of a .Net class, a NullReferenceException is thrown.

A brief snippet to demonstrate the issue:

    class Person { public string FirstName = "a"; }

    class Program
    {
        static void Main(string[] args)
        {
            Person p = new Person();
            
            Engine engine = new Engine();
            engine.SetValue("P", p);

            // Working
            Engine e1 = engine.Execute("P.FirstName = 'b'; ");
            Console.WriteLine(p.FirstName);

            // NullReferenceException
            Engine e2 = engine.Execute("P.FirstName += 'c'; "); // += Operator fails
            Console.WriteLine(p.FirstName);
            
            Console.ReadKey();
        }
    }

[lahma]

Thanks for reporting this, fixed via #630 .