Fix null de-ref in CXFA_FFNotify::OnValueChanged().

Widget doesn't always have a parent.

Bug: 489144038
Change-Id: I9269408e03d916b289492c46376cc7399467d728
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/144310
Reviewed-by: Andy Phan <andyphan@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

xfa/fxfa/cxfa_ffnotify.cpp

diff --git a/xfa/fxfa/cxfa_ffnotify.cpp b/xfa/fxfa/cxfa_ffnotify.cpp
index c16dd77..ea430f2 100644
--- a/xfa/fxfa/cxfa_ffnotify.cpp
+++ b/xfa/fxfa/cxfa_ffnotify.cpp
@@ -453,9 +453,12 @@
         doc_->GetDocView()->UpdateUIDisplay(pWidgetNode, nullptr);
         pDocView->AddCalculateNode(pWidgetNode);
         pDocView->AddValidateNode(pWidgetNode);
-      } else if (pWidgetNode->GetParent()->GetElementType() ==
-                 XFA_Element::ExclGroup) {
-        doc_->GetDocView()->UpdateUIDisplay(pWidgetNode, nullptr);
+      } else {
+        CXFA_Node* widget_parent = pWidgetNode->GetParent();
+        if (widget_parent &&
+            widget_parent->GetElementType() == XFA_Element::ExclGroup) {
+          doc_->GetDocView()->UpdateUIDisplay(pWidgetNode, nullptr);
+        }
       }
       return;
     }