Fix an assertion failure in CPDFSDK_Widget* Field::GetWidget()

fpdfsdk/src/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 "Field.h"
 
 #include "Document.h"
 #include "Icon.h"
@@ skipping 271 matching lines @@
       pDoc->UpdateAllViews(NULL, pWidget);
     }
   }
 
   if (bChangeMark)
     pDocument->SetChangeMark();
 }
 
 CPDFSDK_Widget* Field::GetWidget(CPDFSDK_Document* pDocument,
                                  CPDF_FormControl* pFormControl) {
-  ASSERT(pDocument != NULL);
-  ASSERT(pFormControl != NULL);
-
-  CPDFSDK_InterForm* pInterForm = (CPDFSDK_InterForm*)pDocument->GetInterForm();
-  ASSERT(pInterForm != NULL);
-
-  return pInterForm->GetWidget(pFormControl);
+  CPDFSDK_InterForm* pInterForm =
+      static_cast<CPDFSDK_InterForm*>(pDocument->GetInterForm());
+  return pInterForm ? pInterForm->GetWidget(pFormControl) : nullptr;
 }
 
 FX_BOOL Field::ValueIsOccur(CPDF_FormField* pFormField,
                             CFX_WideString csOptLabel) {
   ASSERT(pFormField != NULL);
 
   for (int i = 0, sz = pFormField->CountOptions(); i < sz; i++) {
     if (csOptLabel.Compare(pFormField->GetOptionLabel(i)) == 0)
       return TRUE;
   }
@@ skipping 3429 matching lines @@
 #define JS_FIELD_MINWIDTH 1
 #define JS_FIELD_MINHEIGHT 1
 
 void Field::AddField(CPDFSDK_Document* pDocument,
                      int nPageIndex,
                      int nFieldType,
                      const CFX_WideString& sName,
                      const CPDF_Rect& rcCoords) {
   // Not supported.
 }