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

pkg/analysis_server/lib/src/edit/edit_domain.dart

Index: pkg/analysis_server/lib/src/edit/edit_domain.dart
diff --git a/pkg/analysis_server/lib/src/edit/edit_domain.dart b/pkg/analysis_server/lib/src/edit/edit_domain.dart
index e848fbe33802977fc5b114036a2e37a6d950a5de..b076c7d18f6d78d1fece4cc436b5d401bc7af98f 100644
--- a/pkg/analysis_server/lib/src/edit/edit_domain.dart
+++ b/pkg/analysis_server/lib/src/edit/edit_domain.dart
@@ -587,8 +587,10 @@ class _RefactoringManager {
    * 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();
   }