My guess would be that the null reference exception is occurring when you set the value of "value" and "display", because SelectSingleNode is failing to select a node. I see that SingleMember is using the "my" namespace - are "value" and "display" children of SingleMember? If they are fields, they'd probably also be using the "my" namespace:
newMember.SelectSingleNode("my:value", NamespaceManager).SetValue("test");
For example (don't forget, you'll need a NamespaceManager for SelectSingleNode, since the XPath has a namespace).