Fix the crash that the previous commit half-fixed.

Fix the crash that the previous commit half-fixed.

studio::App::quit()                                                     at app.cpp:1575
  studio::Instance::safe_close ()                                       at instance.cpp:687
    [ handle magic ]
      ~CanvasView (this=0xa432378)                                      at canvasview.cpp:918
        ~WorkArea (this=0x8ed2a10)                                      at workarea.cpp:815
          [ handle magic ]
        ^   ~Canvas (this=0x8e8a2b8)                                    at canvas.cpp:94
        |     synfig::Canvas::clear ()                                  at canvas.cpp:136
        |       synfig::Canvas::erase (iter)                            at canvas.cpp:758
        |         synfig::Node::changed ()                              at node.cpp:194
        |           synfig::Canvas::on_changed ()                       at canvas.cpp:86
        |             synfig::Node::on_changed ()                       at node.cpp:290
        |               [ sigc magic ]
        |                 studio::CanvasView::on_dirty_preview ()       at canvasview.cpp:2774
        `- illegal -------  work_area->queue_render_preview();

git-svn-id: http://svn.voria.com/code@767 1f10aa63-cdf2-0310-b900-c93c546f37ac