fix NSMError in completion

pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart

Index: pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart b/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
index 76f2bff5a51a45bc3a2b238ce3f6115cf4f31fc2..08f394b137e0ec39fb399034b1c02b64fb8f2ce2 100644
--- a/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
@@ -154,7 +154,7 @@ class _LocalDeclarationFinder extends LocalDeclarationVisitor {
       VariableDeclarationList varList, VariableDeclaration varDecl) {}
 
   void _addArgListSuggestion(FormalParameterList parameters) {
-    if (parameters.parameters.length == 0) {
+    if (parameters == null || parameters.parameters.length == 0) {
       return;
     }
     StringBuffer completion = new StringBuffer('(');