--- a/layout/forms/nsComboboxControlFrame.cpp
+++ b/layout/forms/nsComboboxControlFrame.cpp
@@ -1462,16 +1462,20 @@ nsComboboxControlFrame::SetInitialChildL
 
 //----------------------------------------------------------------------
   //nsIRollupListener
 //----------------------------------------------------------------------
 bool
 nsComboboxControlFrame::Rollup(uint32_t aCount, bool aFlush,
                                const nsIntPoint* pos, nsIContent** aLastRolledUp)
 {
+  if (aLastRolledUp) {
+    *aLastRolledUp = nullptr;
+  }
+
   if (!mDroppedDown) {
     return false;
   }
 
   bool consume = !!COMBOBOX_ROLLUP_CONSUME_EVENT;
   AutoWeakFrame weakFrame(this);
   mListControlFrame->AboutToRollup(); // might destroy us
   if (!weakFrame.IsAlive()) {

--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -200,16 +200,20 @@ nsXULPopupManager::GetInstance()
   MOZ_ASSERT(sInstance);
   return sInstance;
 }
 
 bool
 nsXULPopupManager::Rollup(uint32_t aCount, bool aFlush,
                           const nsIntPoint* pos, nsIContent** aLastRolledUp)
 {
+  if (aLastRolledUp) {
+    *aLastRolledUp = nullptr;
+  }
+
   // We can disable the autohide behavior via a pref to ease debugging.
   if (nsXULPopupManager::sDevtoolsDisableAutoHide) {
     // Required on linux to allow events to work on other targets.
     if (mWidget) {
       mWidget->CaptureRollupEvents(nullptr, false);
     }
     return false;
   }