Fix AppModalDialogHelper to deal with a TabStrip without an active tab

chrome/browser/ui/views/javascript_app_modal_dialog_views_x11.cc

 // Copyright 2014 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/ui/views/javascript_app_modal_dialog_views_x11.h"
 
 #include "chrome/browser/ui/blocked_content/app_modal_dialog_helper.h"
 #include "chrome/browser/ui/views/javascript_app_modal_event_blocker_x11.h"
 #include "components/app_modal/javascript_app_modal_dialog.h"
 #include "ui/views/widget/widget.h"
 
 JavaScriptAppModalDialogViewsX11::JavaScriptAppModalDialogViewsX11(
     app_modal::JavaScriptAppModalDialog* parent)
     : app_modal::JavaScriptAppModalDialogViews(parent),
-      helper_(new AppModalDialogHelper(parent->web_contents())) {
+      helper_(
+          new AppModalDialogHelper(parent ? parent->web_contents() : nullptr)) {
 }
 
 JavaScriptAppModalDialogViewsX11::~JavaScriptAppModalDialogViewsX11() {
 }
 
 void JavaScriptAppModalDialogViewsX11::ShowAppModalDialog() {
   // BrowserView::CanActivate() ensures that other browser windows cannot be
   // activated for long while the dialog is visible. Block events to other
   // browser windows so that the user cannot interact with other browser windows
   // in the short time that the other browser windows are active. This hack is
   // unnecessary on Windows and Chrome OS.
   // TODO(pkotwicz): Find a better way of doing this and remove this hack.
   if (!event_blocker_x11_.get()) {
     event_blocker_x11_.reset(
         new JavascriptAppModalEventBlockerX11(GetWidget()->GetNativeView()));
   }
   GetWidget()->Show();
 }
 
 void JavaScriptAppModalDialogViewsX11::WindowClosing() {
   event_blocker_x11_.reset();
 }