[CSS Regions] Elements in a region should be assignable to a named flow

Source/core/rendering/RenderNamedFlowThread.cpp

Index: Source/core/rendering/RenderNamedFlowThread.cpp
diff --git a/Source/core/rendering/RenderNamedFlowThread.cpp b/Source/core/rendering/RenderNamedFlowThread.cpp
index cefa26fde982ad005711fb441dd864fc46466df0..0f64ff51946c4887036b7f5fcb86dce1fd5f63f6 100644
--- a/Source/core/rendering/RenderNamedFlowThread.cpp
+++ b/Source/core/rendering/RenderNamedFlowThread.cpp
@@ -463,6 +463,9 @@ bool RenderNamedFlowThread::isChildAllowed(RenderObject* child, RenderStyle* sty
 
     ASSERT(child->node()->isElementNode());
     RenderObject* parentRenderer = NodeRenderingContext(child->node()).parentRenderer();
+    if (!parentRenderer)
+        return true;
+
     return parentRenderer->isChildAllowed(child, style);
 }