Handle incomplete rule options gracefully.

pkg/analysis_server/lib/src/services/linter/linter.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library services.src.linter;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/plugin/options.dart';
 import 'package:linter/src/rules.dart';
 import 'package:yaml/yaml.dart';
@@ skipping 22 matching lines @@
       validateRules(rules, reporter);
     }
     return errors;
   }
 
   validateRules(dynamic rules, ErrorReporter reporter) {
     if (rules is YamlList) {
       //TODO(pq): migrate this to a proper API once there is one.
       Iterable<String> registeredLints = ruleRegistry.map((r) => r.name);
       rules.nodes.forEach((YamlNode ruleNode) {
-        if (!registeredLints.contains(ruleNode.value)) {
+        Object value = ruleNode.value;
+        if (value != null && !registeredLints.contains(value)) {
           reporter.reportErrorForSpan(
-              UNDEFINED_LINT_WARNING, ruleNode.span, [ruleNode.value]);
+              UNDEFINED_LINT_WARNING, ruleNode.span, [value]);
         }
       });
     }
   }
 }