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

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

Index: chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.cc
diff --git a/chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.cc b/chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.cc
index 5215f366cf1e8360c9b2e4acf1d82263ad2b62e3..107d712f679fb4eb95f7d361c7b8326d931d0500 100644
--- a/chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.cc
+++ b/chrome/browser/android/offline_pages/downloads/offline_page_download_bridge.cc
@@ -178,6 +178,8 @@ ScopedJavaLocalRef<jobject> OfflinePageDownloadBridge::GetItemByGuid(
     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);
 }