[FastTextAutosizer] Check for containingBlock in deepestBlockContainingAllText

Source/core/rendering/FastTextAutosizer.cpp

Index: Source/core/rendering/FastTextAutosizer.cpp
diff --git a/Source/core/rendering/FastTextAutosizer.cpp b/Source/core/rendering/FastTextAutosizer.cpp
index 2dea427fd5d3e084551eb6a921d051a28f0eab79..a0983990ea7aba85b03aaf5f3a332dbd9a8fc8e3 100644
--- a/Source/core/rendering/FastTextAutosizer.cpp
+++ b/Source/core/rendering/FastTextAutosizer.cpp
@@ -921,8 +921,10 @@ const RenderBlock* FastTextAutosizer::deepestBlockContainingAllText(const Render
     // its text node's lowest common ancestor as isAutosizingCluster would have made them into their
     // own independent cluster.
     const RenderBlock* containingBlock = firstNode->containingBlock();
-    ASSERT(containingBlock->isDescendantOf(root));
+    if (!containingBlock)
+        return root;
 
+    ASSERT(containingBlock->isDescendantOf(root));
     return containingBlock;
 }