Fixes crash in PanelLayoutManager

ash/common/wm/panels/panel_layout_manager.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/common/wm/panels/panel_layout_manager.h"
 
 #include <algorithm>
 #include <map>
 #include <utility>
 
@@ skipping 554 matching lines @@
 
 void PanelLayoutManager::RestorePanel(WmWindow* panel) {
   PanelList::iterator found =
       std::find(panel_windows_.begin(), panel_windows_.end(), panel);
   DCHECK(found != panel_windows_.end());
   found->slide_in = true;
   Relayout();
 }
 
 void PanelLayoutManager::Relayout() {
-  if (!shelf_)
+  if (!shelf_ || !shelf_->GetWindow())
     return;
 
   // Suppress layouts during overview mode because changing window bounds
   // interfered with overview mode animations. However, layouts need to be done
   // when the WindowSelectorController is restoring minimized windows so that
   // they actually become visible.
   WindowSelectorController* window_selector_controller =
       WmShell::Get()->window_selector_controller();
   if (in_layout_ ||
       (window_selector_controller->IsSelecting() &&
@@ skipping 305 matching lines @@
       // Keyboard hidden, restore original bounds if they exist.
       panel->SetBoundsDirect(panel_state->GetRestoreBoundsInScreen());
     }
   }
   // This bounds change will have caused a change to the Shelf which does not
   // propogate automatically to this class, so manually recalculate bounds.
   OnWindowResized();
 }
 
 }  // namespace ash