Don't try to re-layout shelf if the widget has already been deleted.

ash/shelf/shelf_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/shelf/shelf_layout_manager.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "ash/ash_switches.h"
@@ skipping 186 matching lines @@
 
   alignment_ = alignment;
   if (shelf_->launcher())
     shelf_->launcher()->SetAlignment(alignment);
   shelf_->status_area_widget()->SetShelfAlignment(alignment);
   LayoutShelf();
   return true;
 }
 
 gfx::Rect ShelfLayoutManager::GetIdealBounds() {
-  // TODO(oshima): this is wrong. Figure out what display shelf is on
-  // and everything should be based on it.
   gfx::Rect bounds(
       ScreenAsh::GetDisplayBoundsInParent(shelf_->GetNativeView()));
   int width = 0, height = 0;
   GetShelfSize(&width, &height);
   return SelectValueForShelfAlignment(
       gfx::Rect(bounds.x(), bounds.bottom() - height, bounds.width(), height),
       gfx::Rect(bounds.x(), bounds.y(), width, bounds.height()),
       gfx::Rect(bounds.right() - width, bounds.y(), width, bounds.height()),
       gfx::Rect(bounds.x(), bounds.y(), bounds.width(), height));
 }
@@ skipping 288 matching lines @@
   return shelf ? shelf->shelf_layout_manager() : NULL;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // ShelfLayoutManager, private:
 
 ShelfLayoutManager::TargetBounds::TargetBounds() : opacity(0.0f) {}
 ShelfLayoutManager::TargetBounds::~TargetBounds() {}
 
 void ShelfLayoutManager::SetState(ShelfVisibilityState visibility_state) {
+  if (!shelf_->GetNativeView())
+    return;
+
   State state;
   state.visibility_state = visibility_state;
   state.auto_hide_state = CalculateAutoHideState(visibility_state);
   state.is_screen_locked =
       Shell::GetInstance()->session_state_delegate()->IsScreenLocked();
 
   // It's possible for SetState() when a window becomes maximized but the state
   // won't have changed value. Do the dimming check before the early exit.
   if (workspace_controller_) {
     shelf_->SetDimsShelf(
@@ skipping 380 matching lines @@
 int ShelfLayoutManager::GetWorkAreaSize(const State& state, int size) const {
   if (state.visibility_state == SHELF_VISIBLE)
     return size;
   if (state.visibility_state == SHELF_AUTO_HIDE)
     return kAutoHideSize;
   return 0;
 }
 
 }  // namespace internal
 }  // namespace ash