Watch for destruction of widget during callback

fpdfsdk/javascript/Field.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 "fpdfsdk/javascript/Field.h"
 
 #include <algorithm>
 #include <memory>
@@ skipping 210 matching lines @@
                             FX_BOOL bRefresh) {
   std::vector<CPDFSDK_Widget*> widgets;
   CPDFSDK_InterForm* pInterForm = pDocument->GetInterForm();
   pInterForm->GetWidgets(pFormField, &widgets);
 
   if (bResetAP) {
     int nFieldType = pFormField->GetFieldType();
     if (nFieldType == FIELDTYPE_COMBOBOX || nFieldType == FIELDTYPE_TEXTFIELD) {
       for (CPDFSDK_Widget* pWidget : widgets) {
         FX_BOOL bFormatted = FALSE;
+        CPDFSDK_Widget::Observer observer(&pWidget);
         CFX_WideString sValue = pWidget->OnFormat(bFormatted);
-        pWidget->ResetAppearance(bFormatted ? sValue.c_str() : nullptr, FALSE);
+        if (pWidget) {
+          pWidget->ResetAppearance(bFormatted ? sValue.c_str() : nullptr,
+                                   FALSE);
+        }
       }
     } else {
       for (CPDFSDK_Widget* pWidget : widgets) {
         pWidget->ResetAppearance(nullptr, FALSE);
       }
     }
   }
 
   if (bRefresh) {
     for (CPDFSDK_Widget* pWidget : widgets) {
@@ skipping 3288 matching lines @@
   }
 }
 
 void Field::AddField(CPDFSDK_Document* pDocument,
                      int nPageIndex,
                      int nFieldType,
                      const CFX_WideString& sName,
                      const CFX_FloatRect& rcCoords) {
   // Not supported.
 }