Fix exception in code completion (issue 23439)

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

Index: pkg/analysis_server/lib/src/services/completion/local_declaration_visitor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/local_declaration_visitor.dart b/pkg/analysis_server/lib/src/services/completion/local_declaration_visitor.dart
index 6ba0afa8c4440fe019931f7b9761aa369236de90..9032b05bd848d0078d81296fdaefe0f599731354 100644
--- a/pkg/analysis_server/lib/src/services/completion/local_declaration_visitor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/local_declaration_visitor.dart
@@ -160,7 +160,10 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
       id = node.identifier;
       type = null;
     }
-    declaredLocalVar(id, type);
+    if (id != null) {
+      // If there is no loop variable, don't declare it.
+      declaredLocalVar(id, type);
+    }
     visitNode(node);
   }