System.NullReferenceException: Object reference not set to an instance of an object.

[Ryder17z]

C:\Users\ExileFox\Documents\sortxml-develop\bin\Debug>sortxml.exe Cards.dat Cards_XML.xml

Unhandled Exception: System.InvalidOperationException: Failed to compare two elements in the array. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at sortxml.Program.SortDelegate(XmlNode a, XmlNode b) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 196
   at System.Array.FunctorComparer`1.Compare(T x, T y)
   at System.Collections.Generic.ArraySortHelper`1.SwapIfGreater(T[] keys, IComparer`1 comparer, Int32 a, Int32 b)
   at System.Collections.Generic.ArraySortHelper`1.IntroSort(T[] keys, Int32 lo, Int32 hi, Int32 depthLimit, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.IntrospectiveSort(T[] keys, Int32 left, Int32 length, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
   --- End of inner exception stack trace ---
   at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
   at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer`1 comparer)
   at System.Collections.Generic.List`1.Sort(Comparison`1 comparison)
   at sortxml.Program.SortNodes(XmlNode node) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 175
   at sortxml.Program.SortNodes(XmlNode node) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 163
   at sortxml.Program.SortNodes(XmlNode node) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 163
   at sortxml.Program.SortNodes(XmlNode node) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 163
   at sortxml.Program.SortNodes(XmlNode node) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 163
   at sortxml.Program.Main(String[] arguments) in C:\Users\ExileFox\Documents\sortxml-develop\Program.cs:line 129