[Offline pages] Fixing a crash when notification for removed downloaded page is clicked

chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.cc

 // 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 "chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.h"
 
 #include <vector>
 
 #include "base/android/jni_string.h"
 #include "base/bind.h"
@@ skipping 160 matching lines @@
       download_ui_adapter_->GetAllItems();
   ToJavaOfflinePageDownloadItemList(env, j_result_obj, items);
 }
 
 ScopedJavaLocalRef<jobject> OfflinePageDownloadBridge::GetItemByGuid(
     JNIEnv* env,
     const JavaParamRef<jobject>& obj,
     const JavaParamRef<jstring>& j_guid) {
   std::string guid = ConvertJavaStringToUTF8(env, j_guid);
   const DownloadUIItem* item = download_ui_adapter_->GetItem(guid);
+  if (item == nullptr)
+    return ScopedJavaLocalRef<jobject>();
   return ToJavaOfflinePageDownloadItem(env, *item);
 }
 
 void OfflinePageDownloadBridge::DeleteItemByGuid(
     JNIEnv* env,
     const JavaParamRef<jobject>& obj,
     const JavaParamRef<jstring>& j_guid) {
   std::string guid = ConvertJavaStringToUTF8(env, j_guid);
   download_ui_adapter_->DeleteItem(guid);
 }
@@ skipping 159 matching lines @@
 
   DownloadUIAdapter* adapter =
       DownloadUIAdapter::FromOfflinePageModel(offline_page_model);
 
   return reinterpret_cast<jlong>(
       new OfflinePageDownloadBridge(env, obj, adapter, browser_context));
 }
 
 }  // namespace android
 }  // namespace offline_pages