Handle incomplete rule options gracefully.

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

Index: pkg/analysis_server/lib/src/services/linter/linter.dart
diff --git a/pkg/analysis_server/lib/src/services/linter/linter.dart b/pkg/analysis_server/lib/src/services/linter/linter.dart
index 675836cee49a2b164f88c34f460bf4034338d34c..964c238d0487f82764e632848baa60603e9969b8 100644
--- a/pkg/analysis_server/lib/src/services/linter/linter.dart
+++ b/pkg/analysis_server/lib/src/services/linter/linter.dart
@@ -40,9 +40,10 @@ class LinterRuleOptionsValidator extends OptionsValidator {
       //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]);
         }
       });
     }