Value cannot be null. Parameter name: type at System.Activator.CreateInstance(Type type, Boolean nonPublic)

Your first problem in your originally posted code was that in your config you called the property "class", but in your GetType statement, you used "className".

I think this should not be a problem because in config handler I have used the below code:

   [ConfigurationProperty("class", IsKey = true, IsRequired = true)]
public string className
{
           get { 
                    return (string)this["class"]; 
           }
            set { 
                    this["class"] = value; 
            }
}