[Reading List iOS] Test WebState on JavaScript return.

ios/chrome/browser/reading_list/reading_list_distiller_page.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/chrome/browser/reading_list/reading_list_distiller_page.h"
 
 #include "base/bind.h"
 #include "base/mac/foundation_util.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 178 matching lines @@
     // Workaround for Wikipedia pages.
     // TODO(crbug.com/647667): remove workaround once DOM distiller handle this
     // case.
     HandleWikipediaPage();
     return;
   }
   ContinuePageDistillation();
 }
 
 void ReadingListDistillerPage::ContinuePageDistillation() {
+  if (!CurrentWebState()) {
+    // Something interrupted the distillation.
+    // Abort here.
+    return;
+  }
   // The page is ready to be distilled.
   // If the visible URL is not the original URL, notify the caller that URL
   // changed.
   GURL redirected_url = CurrentWebState()->GetVisibleURL();
   if (redirected_url != original_url_ && delegate_) {
     delegate_->DistilledPageRedirectedToURL(original_url_, redirected_url);
   }
   DistillerPageIOS::OnLoadURLDone(web::PageLoadCompletionStatus::SUCCESS);
 }
 
@@ skipping 79 matching lines @@
   [CurrentWebState()->GetJSInjectionReceiver()
       executeJavaScript:@(kWikipediaWorkaround)
       completionHandler:^(id result, NSError* error) {
         if (weak_this) {
           weak_this->ContinuePageDistillation();
         }
       }];
 }
 
 }  // namespace reading_list