Catch null return value from computeComputedStyle.

third_party/WebKit/Source/core/css/CSSComputedStyleDeclaration.cpp

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2011 Sencha, Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
@@ skipping 513 matching lines @@
         return nullptr;
     if (m_node->isElementNode()) {
         if (PseudoElement* element = toElement(m_node)->pseudoElement(m_pseudoElementSpecifier))
             return element;
     }
     return m_node.get();
 }
 
 PassRefPtrWillBeRawPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValue(AtomicString customPropertyName) const
 {
-    return ComputedStyleCSSValueMapping::get(customPropertyName, *computeComputedStyle());
+    const ComputedStyle* style = computeComputedStyle();
+    if (!style)
+        return nullptr;
+    return ComputedStyleCSSValueMapping::get(customPropertyName, *style);
 }
 
 const HashMap<AtomicString, RefPtr<CSSVariableData>>* CSSComputedStyleDeclaration::getVariables() const
 {
-    return ComputedStyleCSSValueMapping::getVariables(*computeComputedStyle());
+    const ComputedStyle* style = computeComputedStyle();
+    if (!style)
+        return nullptr;
+    return ComputedStyleCSSValueMapping::getVariables(*style);
 }
 
 PassRefPtrWillBeRawPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValue(CSSPropertyID propertyID) const
 {
     Node* styledNode = this->styledNode();
     if (!styledNode)
         return nullptr;
     LayoutObject* layoutObject = styledNode->layoutObject();
     const ComputedStyle* style;
 
@@ skipping 150 matching lines @@
     exceptionState.throwDOMException(NoModificationAllowedError, "These styles are computed, and therefore the '" + getPropertyNameString(id) + "' property is read-only.");
 }
 
 DEFINE_TRACE(CSSComputedStyleDeclaration)
 {
     visitor->trace(m_node);
     CSSStyleDeclaration::trace(visitor);
 }
 
 } // namespace blink