Guard against missing enclosing class (issue 28163)

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

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 52fe2130b214f9c8a3b17accfd5ff0301c77fd64..f0b353fa6ebe28abbfca0d4f79470f71a8927796 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -1659,6 +1659,9 @@ class FixProcessor {
   void _addFix_makeEnclosingClassAbstract() {
     ClassDeclaration enclosingClass =
         node.getAncestor((node) => node is ClassDeclaration);
+    if (enclosingClass == null) {
+      return;
+    }
     String className = enclosingClass.name.name;
     _addInsertEdit(enclosingClass.classKeyword.offset, 'abstract ');
     _addFix(DartFixKind.MAKE_CLASS_ABSTRACT, [className]);