Fix critical warning when destroying a PeasPluginManagerView.
Fix critical warning when destroying a PeasPluginManagerView.

The 'cursor-changed' signal is now (since gtk+ 3.4) emitted during
the destruction of GtkTreeView, which leads us to call
gtk_tree_view_get_selection() after the selection object has been freed,
hence receiving a NULL we weren't expecting before.

Distrotech/libpeas,gregier/libpeas,GNOME/libpeas,chergert/libpeas,chergert/libpeas,Distrotech/libpeas,Distrotech/libpeas,gregier/libpeas,GNOME/libpeas,gregier/libpeas,chergert/libpeas,gregier/libpeas