Issue 20890. Fix for NPE in the 'Add show' QA.

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/assist_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
index 5280a44f99de21cc2e27f6101751ee38c45492b9..186c47888242ca59b203c5218d6e3bb40eacbc94 100644
--- a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
@@ -530,7 +530,7 @@ class AssistProcessor {
     _SimpleIdentifierRecursiveAstVisitor visitor =
         new _SimpleIdentifierRecursiveAstVisitor((SimpleIdentifier node) {
       Element element = node.staticElement;
-      if (namespace[node.name] == element) {
+      if (element != null && namespace[node.name] == element) {
         referencedNames.add(element.displayName);
       }
     });