Prevent an exception (issue 30307)

pkg/analyzer/lib/src/dart/constant/evaluation.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 analyzer.src.dart.constant.evaluation;
 
 import 'dart:collection';
 
 import 'package:analyzer/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 573 matching lines @@
     List<FieldElement> fields = constructor.enclosingElement.fields;
     for (int i = 0; i < fields.length; i++) {
       FieldElement field = fields[i];
       if ((field.isFinal || field.isConst) &&
           !field.isStatic &&
           field is ConstFieldElementImpl) {
         validator.beforeGetFieldEvaluationResult(field);
 
         DartObjectImpl fieldValue;
         if (strongMode) {
-          fieldValue = field.constantInitializer.accept(fieldInitVisitor);
+          fieldValue = field.constantInitializer?.accept(fieldInitVisitor);
         } else {
           fieldValue = field.evaluationResult?.value;
         }
         // It is possible that the evaluation result is null.
         // This happens for example when we have duplicate fields.
         // class Test {final x = 1; final x = 2; const Test();}
         if (fieldValue == null) {
           continue;
         }
         // Match the value and the type.
@@ skipping 1525 matching lines @@
   }
 
   @override
   String toString() {
     if (value == null) {
       return "error";
     }
     return value.toString();
   }
 }