Watch for destruction of widget during callback

fpdfsdk/fsdk_baseform.cpp

Index: fpdfsdk/fsdk_baseform.cpp
diff --git a/fpdfsdk/fsdk_baseform.cpp b/fpdfsdk/fsdk_baseform.cpp
index afbd95870d3f62b5ed0842ca6e7a99abda1fc4fa..dbaa06011d54cee8f8fddbae005bde422ba4ffa8 100644
--- a/fpdfsdk/fsdk_baseform.cpp
+++ b/fpdfsdk/fsdk_baseform.cpp
@@ -26,6 +26,7 @@
 #include "fpdfsdk/javascript/ijs_context.h"
 #include "fpdfsdk/javascript/ijs_runtime.h"
 #include "fpdfsdk/pdfwindow/PWL_Utils.h"
+#include "third_party/base/stl_util.h"
 
 #ifdef PDF_ENABLE_XFA
 #include "fpdfsdk/fpdfxfa/include/fpdfxfa_doc.h"
@@ -47,6 +48,22 @@ PDFSDK_FieldAction::PDFSDK_FieldAction()
       bFieldFull(FALSE),
       bRC(TRUE) {}
 
+CPDFSDK_Widget::Observer::Observer(CPDFSDK_Widget** pWatchedPtr)
+    : m_pWatchedPtr(pWatchedPtr) {
+  (*m_pWatchedPtr)->AddObserver(this);
+}
+
+CPDFSDK_Widget::Observer::~Observer() {
+  if (m_pWatchedPtr)
+    (*m_pWatchedPtr)->RemoveObserver(this);
+}
+
+void CPDFSDK_Widget::Observer::OnWidgetDestroyed() {
+  ASSERT(m_pWatchedPtr);
+  *m_pWatchedPtr = nullptr;
+  m_pWatchedPtr = nullptr;
+}
+
 CPDFSDK_Widget::CPDFSDK_Widget(CPDF_Annot* pAnnot,
                                CPDFSDK_PageView* pPageView,
                                CPDFSDK_InterForm* pInterForm)
@@ -62,7 +79,20 @@ CPDFSDK_Widget::CPDFSDK_Widget(CPDF_Annot* pAnnot,
 {
 }
 
-CPDFSDK_Widget::~CPDFSDK_Widget() {}
+CPDFSDK_Widget::~CPDFSDK_Widget() {
+  for (auto* pObserver : m_Observers)
+    pObserver->OnWidgetDestroyed();
+}
+
+void CPDFSDK_Widget::AddObserver(Observer* pObserver) {
+  ASSERT(!pdfium::ContainsKey(m_Observers, pObserver));
+  m_Observers.insert(pObserver);
+}
+
+void CPDFSDK_Widget::RemoveObserver(Observer* pObserver) {
+  ASSERT(pdfium::ContainsKey(m_Observers, pObserver));
+  m_Observers.erase(pObserver);
+}
 
 #ifdef PDF_ENABLE_XFA
 CXFA_FFWidget* CPDFSDK_Widget::GetMixXFAWidget() const {