Null-check scrollable area when invalidating sticky constraints.

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 383 matching lines @@
         // scroll reasons are recomputed.
       } else {
         // This may get re-added to viewport constrained objects if the object
         // went from sticky to fixed.
         frameView->removeViewportConstrainedObject(this);
 
         // Remove sticky constraints for this layer.
         if (layer()) {
           DisableCompositingQueryAsserts disabler;
           if (const PaintLayer* ancestorOverflowLayer =
-                  layer()->ancestorOverflowLayer())
-            ancestorOverflowLayer->getScrollableArea()
-                ->invalidateStickyConstraintsFor(layer());
+                  layer()->ancestorOverflowLayer()) {
+            if (PaintLayerScrollableArea* scrollableArea =
+                    ancestorOverflowLayer->getScrollableArea())
+              scrollableArea->invalidateStickyConstraintsFor(layer());
+          }
         }
 
         // TODO(pdr): When slimming paint v2 is enabled, we will need to
         // invalidate the scroll paint property subtree for this so main thread
         // scroll reasons are recomputed.
       }
     }
 
     if (newStyleIsViewportConstained != oldStyleIsViewportConstrained) {
       if (newStyleIsViewportConstained && layer())
@@ skipping 921 matching lines @@
   if (rootElementStyle->hasBackground())
     return false;
 
   if (node() != document().firstBodyElement())
     return false;
 
   return true;
 }
 
 }  // namespace blink