Get rid of NULLs in xfa/

xfa/fwl/core/fwl_appimp.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "xfa/fwl/core/fwl_appimp.h"
 
 #include "xfa/fwl/core/cfwl_widgetmgr.h"
 #include "xfa/fwl/core/fwl_noteimp.h"
@@ skipping 81 matching lines @@
   }
 }
 
 IFWL_ThemeProvider* CFWL_AppImp::GetThemeProvider() const {
   return m_pThemeProvider;
 }
 
 CXFA_FFApp* FWL_GetAdapterNative() {
   IFWL_App* pApp = FWL_GetApp();
   if (!pApp)
-    return NULL;
+    return nullptr;
   return pApp->GetAdapterNative();
 }
 
 static IFWL_App* g_theApp = nullptr;
 IFWL_App* FWL_GetApp() {
   return g_theApp;
 }
 
 void FWL_SetApp(IFWL_App* pApp) {
   g_theApp = pApp;
 }