--- a/layout/xul/tree/nsTreeSelection.cpp
+++ b/layout/xul/tree/nsTreeSelection.cpp
@@ -291,16 +291,19 @@ NS_IMETHODIMP nsTreeSelection::TimedSele
   nsresult rv = Select(aIndex);
   if (NS_FAILED(rv)) return rv;
 
   if (aMsec != -1) {
     mSuppressed = suppressSelect;
     if (!mSuppressed) {
       if (mSelectTimer) mSelectTimer->Cancel();
 
+      if (!mTree) {
+        return NS_ERROR_UNEXPECTED;
+      }
       nsIEventTarget* target =
           mTree->OwnerDoc()->EventTargetFor(TaskCategory::Other);
       NS_NewTimerWithFuncCallback(getter_AddRefs(mSelectTimer), SelectCallback,
                                   this, aMsec, nsITimer::TYPE_ONE_SHOT,
                                   "nsTreeSelection::SelectCallback", target);
     }
   }