Automapper extension to IGNORE unmapped properties for a given type

AutoMapperExtensions.cs

    public static class AutoMapperExtensions
    {
        public static IMappingExpression<TSource, TDestination>
          IgnoreAllNonExisting<TSource, TDestination>(this IMappingExpression<TSource, TDestination> expression)
        {
            var sourceType = typeof(TSource);
            var destinationType = typeof(TDestination);
            var existingMaps = Mapper.GetAllTypeMaps().First(x => x.SourceType.Equals(sourceType) && x.DestinationType.Equals(destinationType));
            foreach (var property in existingMaps.GetUnmappedPropertyNames())
            {
                expression.ForMember(property, opt => opt.Ignore());
            }
            return expression;
        }
    }

    // USAGE: 

    Mapper.CreateMap<SourceType, DestinationType>()
                .ForMember(prop => x.Property, opt => opt.MapFrom(src => src.OtherProperty))
                .IgnoreAllNonExisting();

FROM schdr on stackoverflow. From this answer: http://stackoverflow.com/a/6474397/12706

How can i ignore as per below.

public class User
{
public string FirstName{get;set;}
public string LastName{get;set;}
public string EmailAddress{get;set;}
public virtual Employer Employer { get; set; }

}
public class Employer
{
public int EmployerId { get; set; }
public int UserID { get; set; }
public string EmployerName { get; set; }
public string EmployerPhone { get; set; }
public virtual User User { get; set; }
}

How to ignore EmployerId in auto mapping?

This is a very useful extension for working with types with many properties. Unfortunately, it doesn't work with types that inherit from a base class (line 11 throws a NullReferenceException). One workaround is to provide explicit mappings or ignores for base class properties.

Above extension is not working with Automapper 5.0.2