Don't call moveParagraph() with null |VisiblePositon|

Source/core/editing/IndentOutdentCommand.cpp

 /*
  * Copyright (C) 2006, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 167 matching lines @@
     }
     Node* enclosingBlockFlow = enclosingBlock(visibleStartOfParagraph.deepEquivalent().deprecatedNode());
     RefPtrWillBeRawPtr<Node> splitBlockquoteNode = enclosingNode;
     if (enclosingBlockFlow != enclosingNode)
         splitBlockquoteNode = splitTreeToNode(enclosingBlockFlow, enclosingNode, true);
     else {
         // We split the blockquote at where we start outdenting.
         Node* highestInlineNode = highestEnclosingNodeOfType(visibleStartOfParagraph.deepEquivalent(), isInline, CannotCrossEditingBoundary, enclosingBlockFlow);
         splitElement(toElement(enclosingNode), (highestInlineNode) ? highestInlineNode : visibleStartOfParagraph.deepEquivalent().deprecatedNode());
     }
+    VisiblePosition startOfParagraphToMove(startOfParagraph(visibleStartOfParagraph));
+    VisiblePosition endOfParagraphToMove(endOfParagraph(visibleEndOfParagraph));
+    if (startOfParagraphToMove.isNull() || endOfParagraphToMove.isNull())
+        return;
     RefPtrWillBeRawPtr<Node> placeholder = createBreakElement(document());
     insertNodeBefore(placeholder, splitBlockquoteNode);
-    moveParagraph(startOfParagraph(visibleStartOfParagraph), endOfParagraph(visibleEndOfParagraph), VisiblePosition(positionBeforeNode(placeholder.get())), true);
+    moveParagraph(startOfParagraphToMove, endOfParagraphToMove, VisiblePosition(positionBeforeNode(placeholder.get())), true);
 }
 
 // FIXME: We should merge this function with ApplyBlockElementCommand::formatSelection
 void IndentOutdentCommand::outdentRegion(const VisiblePosition& startOfSelection, const VisiblePosition& endOfSelection)
 {
     VisiblePosition endOfCurrentParagraph = endOfParagraph(startOfSelection);
     VisiblePosition endOfLastParagraph = endOfParagraph(endOfSelection);
 
     if (endOfCurrentParagraph == endOfLastParagraph) {
         outdentParagraph();
@@ skipping 36 matching lines @@
 
 void IndentOutdentCommand::formatRange(const Position& start, const Position& end, const Position&, RefPtrWillBeRawPtr<Element>& blockquoteForNextIndent)
 {
     if (tryIndentingAsListItem(start, end))
         blockquoteForNextIndent = nullptr;
     else
         indentIntoBlockquote(start, end, blockquoteForNextIndent);
 }
 
 }