Their can be multiple possibilities for universal error known as
"Object reference not set to an instance of an object". :)
Check order of your nodes of XML matching with order of your
DataContract
Class properties. Because it wont deserialize if order is incorrect.
You can also have a look at my below Tip/Trick on the topic
"Debug WCF REST Service".
Debug WCF REST Service.
Hope it should help to resolve your error.
Cheers - Rais