Make sure observers are not nullptr

ui/views/widget/widget.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 "ui/views/widget/widget.h"
 
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/trace_event/trace_event.h"
@@ skipping 371 matching lines @@
 
 gfx::NativeView Widget::GetNativeView() const {
   return native_widget_->GetNativeView();
 }
 
 gfx::NativeWindow Widget::GetNativeWindow() const {
   return native_widget_->GetNativeWindow();
 }
 
 void Widget::AddObserver(WidgetObserver* observer) {
+  CHECK(observer);

[danakj, 2015/04/06 17:18:47]

please add a TODO and point at a bug you're trying to find. then we can remove
this once it is resolved.

   observers_.AddObserver(observer);
 }
 
 void Widget::RemoveObserver(WidgetObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
 bool Widget::HasObserver(const WidgetObserver* observer) const {
   return observers_.HasObserver(observer);
 }
@@ skipping 1135 matching lines @@
 
 ////////////////////////////////////////////////////////////////////////////////
 // internal::NativeWidgetPrivate, NativeWidget implementation:
 
 internal::NativeWidgetPrivate* NativeWidgetPrivate::AsNativeWidgetPrivate() {
   return this;
 }
 
 }  // namespace internal
 }  // namespace views