Program Dia crashes when deleting a diagram object

* Ubuntu Version: 7.04(Feisty)
* Dia Version: 0.96-pre7-0ubuntu1
* Steps to reproduce:
  1. Open Dia Diagram Editor
  2. Choose any of the diagram objects from the "Diagram Editor" window and drop it into the client drawing area
  3. Select the diagram object that you have dropped, and click "Delete" or select "Delete" from the Edit menu, then Dia crashes without warning.

[Full Stack Trace]:
*** glibc detected *** dia: free(): invalid pointer: 0x0955dd70 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6[0xb72347ad]
/lib/tls/i686/cmov/libc.so.6(cfree+0x90)[0xb7237e10]
/usr/lib/libglib-2.0.so.0(g_free+0x31)[0xb74eb131]
dia(diagram_unselect_object+0x2d)[0x805f52d]
dia(diagram_unselect_objects+0x77)[0x805f607]
dia[0x8066c3e]
dia(edit_delete_callback+0x42)[0x806d722]
/usr/lib/libgobject-2.0.so.0(g_cclosure_marshal_VOID__VOID+0x49)[0xb75639d9]
/usr/lib/libgobject-2.0.so.0(g_closure_invoke+0x12b)[0xb755662b]
/usr/lib/libgobject-2.0.so.0[0xb7567103]
/usr/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x8c7)[0xb7568627]
/usr/lib/libgobject-2.0.so.0(g_signal_emit+0x29)[0xb75687e9]
/usr/lib/libgtk-x11-2.0.so.0(_gtk_action_emit_activate+0x55)[0xb79af9d5]
/usr/lib/libgtk-x11-2.0.so.0[0xb79b1da4]
/usr/lib/libgobject-2.0.so.0(g_closure_invoke+0x12b)[0xb755662b]
/usr/lib/libgobject-2.0.so.0[0xb7567103]
/usr/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x68f)[0xb75683ef]
/usr/lib/libgobject-2.0.so.0(g_signal_emit+0x29)[0xb75687e9]
/usr/lib/libgtk-x11-2.0.so.0(gtk_accel_group_activate+0xd9)[0xb79ab899]
/usr/lib/libgtk-x11-2.0.so.0(gtk_accel_groups_activate+0xfa)[0xb79ab99a]
/usr/lib/libgtk-x11-2.0.so.0(gtk_window_activate_key+0xda)[0xb7bb8e5a]
/usr/lib/libgtk-x11-2.0.so.0[0xb7bb8edc]
/usr/lib/libgtk-x11-2.0.so.0(_gtk_marshal_BOOLEAN__BOXED+0x60)[0xb7a916b0]
/usr/lib/libgobject-2.0.so.0[0xb7554e49]
/usr/lib/libgobject-2.0.so.0(g_closure_invoke+0x12b)[0xb755662b]
/usr/lib/libgobject-2.0.so.0[0xb7567753]
/usr/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x68f)[0xb75683ef]
/usr/lib/libgobject-2.0.so.0(g_signal_emit+0x29)[0xb75687e9]
/usr/lib/libgtk-x11-2.0.so.0[0xb7ba5e18]
/usr/lib/libgtk-x11-2.0.so.0(gtk_propagate_event+0x1ba)[0xb7a8a9fa]
/usr/lib/libgtk-x11-2.0.so.0(gtk_main_do_event+0x317)[0xb7a8bbc7]
/usr/lib/libgdk-x11-2.0.so.0[0xb790d12a]
/usr/lib/libglib-2.0.so.0(g_main_context_dispatch+0x182)[0xb74e3df2]
/usr/lib/libglib-2.0.so.0[0xb74e6dcf]
/usr/lib/libglib-2.0.so.0(g_main_loop_run+0x1a9)[0xb74e7179]
/usr/lib/libgtk-x11-2.0.so.0(gtk_main+0xb4)[0xb7a8c044]
dia(main+0x3f)[0x809ff3f]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xdc)[0xb71e2ebc]
dia[0x805ed11]
======= Memory map: ========
08048000-080b5000 r-xp 00000000 08:08 8898020 /usr/bin/dia-gnome
080b5000-080b9000 rw-p 0006c000 08:08 8898020 /usr/bin/dia-gnome
080b9000-0981b000 rw-p 080b9000 00:00 0 [heap]
b4a00000-b4a21000 rw-p b4a00000 00:00 0
b4a21000-b4b00000 ---p b4a21000 00:00 0
b4ba4000-b4baf000 r-xp 00000000 08:08 1114932 /lib/libgcc_s.so.1
b4baf000-b4bb0000 rw-p 0000a000 08:08 1114932 /lib/libgcc_s.so.1
b4bc5000-b4bcb000 r--p 00000000 08:08 9379138 /usr/share/locale-langpack/zh_CN/LC_MESSAGES/scim.mo
b4bcb000-b4bd1000 r-xp 00000000 08:08 8900749 /usr/lib/gtk-2.0/2.10.0/immodules/im-xim.so
b4bd1000-b4bd2000 rw-p 00005000 08:08 8900749 /usr/lib/gtk-2.0/2.10.0/immodules/im-xim.so
b4bd2000-b4dc8000 r--p 00000000 08:08 9240690 /usr/share/icons/hicolor/icon-theme.cache
b4dc8000-b5471000 r--p 00000000 08:08 9242080 /usr/share/icons/gnome/icon-theme.cache
b5471000-b61a6000 r--p 00000000 08:08 6849006 /usr/share/fonts/VeraSansYuanTi/VeraSansYuanTi-Regular.ttf
b61a6000-b61d3000 r-xp 00000000 08:08 36757 /usr/lib/dia/libvdx_filter.so
b61d3000-b61d4000 rw-p 0002d000 08:08 36757 /usr/lib/dia/libvdx_filter.so
b61d4000-b6207000 r-xp 00000000 08:08 8901968 /usr/lib/libxslt.so.1.1.20
b6207000-b6208000 rw-p 00032000 08:08 8901968 /usr/lib/libxslt.so.1.1.20
b6208000-b620e000 r-xp 00000000 08:08 8900454 /usr/lib/gtk-2.0/2.10.0/loaders/libpixbufloader-xpm.so
b620e000-b620f000 rw-p 00005000 08:08 8900454 /usr/lib/gtk-2.0/2.10.0/loaders/libpixbufloader-xpm.so
b620f000-b6215000 r--p 00000000 08:08 9373591 /usr/share/locale-langpack/zh_CN/LC_MESSAGES/libgnomeui-2.0.mo
b6215000-b621c000 r-xp 00000000 08:08 36755 /usr/lib/dia/libpgf_filter.so
b621c000-b621d000 rw-p 00006000 08:08 36755 /usr/lib/dia/libpgf_filter.so
b621d000-b6227000 r-Ignored (core dumped)