Fix bug in DocumentMarkerController::ShowMarkers()

third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp

Index: third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp
diff --git a/third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp b/third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp
index be31df135c58bc99e68eb65a4d00a4295d3f1890..377c8ee822b2b22f250404de69d4a1b6cfe12615 100644
--- a/third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp
+++ b/third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp
@@ -671,6 +671,9 @@ void DocumentMarkerController::ShowMarkers() const {
     MarkerLists* markers = markers_.at(node);
     for (DocumentMarker::MarkerType type : DocumentMarker::AllMarkers()) {
       DocumentMarkerList* const list = ListForType(markers, type);
+      if (!list)
+        continue;
+
       const HeapVector<Member<DocumentMarker>>& markers_in_list =
           list->GetMarkers();
       for (const DocumentMarker* marker : markers_in_list) {