XFA: Fix a crashier in SetFocus()

fpdfsdk/include/fsdk_mgr.h

Index: fpdfsdk/include/fsdk_mgr.h
diff --git a/fpdfsdk/include/fsdk_mgr.h b/fpdfsdk/include/fsdk_mgr.h
index e53be1b1a51d4b9c266487390344058fa28f12ef..cb3b52d2b4b96444605e61be056a94bc4cfb984f 100644
--- a/fpdfsdk/include/fsdk_mgr.h
+++ b/fpdfsdk/include/fsdk_mgr.h
@@ -450,6 +450,10 @@ class CPDFDoc_Environment final {
     }
     return L"";
   }
+  void FFI_PageEvent(int iPageIndex, int iEventType) const {
+    // Todo: call a call-back function when it's implemented
+    // in applicaiton's side.
+  }
 #endif  // PDF_ENABLE_XFA
 
   FX_BOOL IsJSInitiated() const { return m_pInfo && m_pInfo->m_pJsPlatform; }