fixed another NRE

Author: nosami

MonoDevelop.FSharpBinding/FSharpResolverProvider.fs

@@ -43,7 +43,8 @@ type FSharpResolverProvider() =
 
                 let results =
                     asyncMaybe {
-                        let! tyRes = languageService.GetTypedParseResultWithTimeout (doc.Project.FileName.ToString(), filename, 0, docText, AllowStaleResults.MatchingSource, obsoleteCheck=isObsolete)
+                        let projectFile = doc.Project |> function null -> filename | project -> project.FileName.ToString()
+                        let! tyRes = languageService.GetTypedParseResultWithTimeout (projectFile, filename, 0, docText, AllowStaleResults.MatchingSource, obsoleteCheck=isObsolete)
                         LoggingService.LogDebug "ResolverProvider: Getting declaration location"
                         // Get the declaration location from the language service
                         let line, col, lineStr = doc.Editor.GetLineInfoFromOffset offset