Don't send 'Response.refactoringRequestCancelle' if the request is null.

pkg/analysis_server/lib/src/edit/edit_domain.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 edit.domain;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analysis_server/plugin/edit/assist/assist_dart.dart';
@@ skipping 569 matching lines @@
         refactoring is ExtractMethodRefactoring ||
         refactoring is InlineMethodRefactoring ||
         refactoring is MoveFileRefactoring ||
         refactoring is RenameRefactoring;
   }
 
   /**
    * Cancels processing of the current request and cleans up.
    */
   void cancel() {
-    server.sendResponse(new Response.refactoringRequestCancelled(request));
-    request = null;
+    if (request != null) {
+      server.sendResponse(new Response.refactoringRequestCancelled(request));
+      request = null;
+    }
     _reset();
   }
 
   void getRefactoring(Request _request) {
     // prepare for processing the request
     request = _request;
     result = new EditGetRefactoringResult(
         EMPTY_PROBLEM_LIST, EMPTY_PROBLEM_LIST, EMPTY_PROBLEM_LIST);
     // process the request
     var params = new EditGetRefactoringParams.fromRequest(_request);
@@ skipping 367 matching lines @@
     }
     return new RefactoringStatus();
   }
 }
 
 /**
  * [_RefactoringManager] throws instances of this class internally to stop
  * processing in a manager that was reset.
  */
 class _ResetError {}