Null-check scrollable area when invalidating sticky constraints.

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

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index 4eddf7c1c9d089d45a10745960d4f459806d3437..f2bc1ae20e51797636e1b1740a594968213efb65 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -401,9 +401,11 @@ void LayoutBoxModelObject::styleDidChange(StyleDifference diff,
         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