Fix crash when .mhtml/.pdf files are opened from download shelf on Chrome OS.

chrome/browser/chromeos/file_manager/file_browser_handlers.cc

Index: chrome/browser/chromeos/file_manager/file_browser_handlers.cc
diff --git a/chrome/browser/chromeos/file_manager/file_browser_handlers.cc b/chrome/browser/chromeos/file_manager/file_browser_handlers.cc
index eef6bd75850ff8e8493ed0f0236b7325bd61c239..2cb3ecb5b3e387f99dfb4a264efa15c8af6c04a7 100644
--- a/chrome/browser/chromeos/file_manager/file_browser_handlers.cc
+++ b/chrome/browser/chromeos/file_manager/file_browser_handlers.cc
@@ -480,9 +480,11 @@ bool ExecuteFileBrowserHandler(
   // files to be directly opened with the browser.
   if (ShouldBeOpenedWithBrowser(extension->id(), action_id)) {
     const bool result = OpenFilesWithBrowser(profile, file_urls);
-    done.Run(result
-                 ? extensions::api::file_browser_private::TASK_RESULT_OPENED
-                 : extensions::api::file_browser_private::TASK_RESULT_FAILED);
+    if (!done.is_null()) {
+      done.Run(result
+                   ? extensions::api::file_browser_private::TASK_RESULT_OPENED
+                   : extensions::api::file_browser_private::TASK_RESULT_FAILED);
+    }
     return result;
   }