Replace null with ErrorResolveResult.UnknownError instead of throwing an exception.
Update to NRefactory commit ef1a5d63.

ef1a5d6 Fixed a potential null reference exception in completion.
6188c05 Fixed bug in name proposal completion.
a8a5ed1 Fix NullReferenceException in code completion.
cef9a79 Fix file headers.
0ecb7f4 Fixed bug in CS0169FieldIsNeverUsedIssue
552443a Implemented 'CS0169FieldIsNeverUsedIssue'.
cdfcc33 Added new formatting option: "SpaceBeforeSemicolon".