Fix AppModalDialogHelper to deal with a TabStrip without an active tab

BUG=444540

Review URL: https://codereview.chromium.org/819083002

Cr-Commit-Position: refs/heads/master@{#309425}

chrome/browser/ui/blocked_content/app_modal_dialog_helper.cc

@@ -23,6 +23,7 @@ AppModalDialogHelper::AppModalDialogHelper(content::WebContents* dialog_host)
   content::WebContents* active_web_contents =
       active_browser->tab_strip_model()->GetActiveWebContents();
   if (active_browser->is_type_popup() &&
+      active_web_contents &&
       active_web_contents->GetOpener() == dialog_host) {
     // It's indeed a popup from the dialog opening WebContents. Store it, so
     // we can focus it later.