Steps to reproduce:
1. enable the last.fm plugin with a wrong password or without internet connection
2. start playing some tracks (which will go into audioscrobbler.queue file)
3. close the rhythmbox window, the bug buddy window appears


Stack trace:
(21:48:21) [0x80fa028] [rb_shell_quit] rb-shell.c:2457: Quitting
(21:48:21) [0x80fa028] [rb_shell_player_set_playing_source_internal] rb-shell-player.c:2732: setting playing source to (nil)
(21:48:21) [0x80fa028] [rb_shell_player_set_playing_source_internal] rb-shell-player.c:2755: source is already playing, stopping it
(21:48:21) [0x80fa028] [rb_shell_player_stop] rb-shell-player.c:2800: stopping
(21:48:21) [0x80fa028] [rb_shell_player_sync_with_source] rb-shell-player.c:2563: playing source: (nil), active entry: (nil)
(21:48:21) [0x80fa028] [rb_shell_set_window_title] rb-shell.c:2079: clearing title
(21:48:21) [0x80fa028] [rb_shell_player_get_playing_song_duration] rb-shell-player.c:2926: Did not get playing entry : return -1 as length
(21:48:21) [0x80fa028] [rb_shell_player_get_playing_song_duration] rb-shell-player.c:2926: Did not get playing entry : return -1 as length
(21:48:21) [0x80fa028] [rb_header_sync] rb-header.c:355: syncing with entry = (nil)
(21:48:21) [0x80fa028] [rb_header_sync] rb-header.c:426: not playing
(21:48:21) [0x80fa028] [rb_shell_player_get_playing_song_duration] rb-shell-player.c:2926: Did not get playing entry : return -1 as length
(21:48:21) [0x80fa028] [rb_lastfm_song_changed_cb] rb-lastfm-source.c:1320: non-lastfm entry being played
(21:48:21) [0x80fa028] [rb_shell_player_sync_buttons] rb-shell-player.c:2666: syncing with source 0x83d2d80
(21:48:21) [0x80fa028] [rb_shell_player_sync_with_source] rb-shell-player.c:2563: playing source: (nil), active entry: (nil)
(21:48:21) [0x80fa028] [rb_shell_set_window_title] rb-shell.c:2079: clearing title
(21:48:21) [0x80fa028] [rb_shell_player_get_playing_song_duration] rb-shell-player.c:2926: Did not get playing entry : return -1 as length
(21:48:21) [0x80fa028] [rb_shell_player_get_playing_song_duration] rb-shell-player.c:2926: Did not get playing entry : return -1 as length
(21:48:21) [0x80fa028] [rb_header_sync] rb-header.c:355: syncing with entry = (nil)
(21:48:21) [0x80fa028] [rb_header_sync] rb-header.c:426: not playing
(21:48:21) [0x80fa028] [rb_shell_player_sync_buttons] rb-shell-player.c:2666: syncing with source 0x83d2d80
(21:48:21) [0x80fa028] [rb_shell_playing_source_changed_cb] rb-shell.c:1889: playing source changed
(21:48:21) [0x80fa028] [rb_shell_source_deleted_cb] rb-shell.c:1855: source deleted
(21:48:21) [0x80fa028] [rb_source_dispose] rb-source.c:304: Disposing source 0x81bf240: 'Last.fm'
(21:48:21) [0x80fa028] [rb_lastfm_source_finalize] rb-lastfm-source.c:297: finalizing lastfm source
(21:48:21) [0x80fa028] [rb_source_finalize] rb-source.c:331: Finalizing source 0x81bf240: 'Last.fm'
(21:48:21) [0x80fa028] [rb_source_finalize] rb-source.c:334: Unreffing model 0x847dd50 count: 1
(21:48:21) [0x80fa028] [rhythmdb_query_model_dispose] rhythmdb-query-model.c:633: disposing query model 0x847dd50
(21:48:21) [0x80fa028] [rhythmdb_query_model_finalize] rhythmdb-query-model.c:681: finalizing query model 0x847dd50
(21:48:21) [0x80fa028] [rb_audioscrobbler_finalize] rb-audioscrobbler.c:393: Finalizing Audioscrobbler
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Last.fm
(21:48:21) [0x80fa028] [rb_audioscrobbler_plugin_finalize] rb-audioscrobbler-plugin.c:100: RBAudioscrobblerPlugin finalising
(21:48:21) [0x80fa028] [rb_shell_source_deleted_cb] rb-shell.c:1855: source deleted
(21:48:21) [0x80fa028] [rb_source_dispose] rb-source.c:304: Disposing source 0x8750820: 'Radio'
(21:48:21) [0x80fa028] [rhythmdb_property_model_dispose] rhythmdb-property-model.c:451: disposing property model 0x8714520
(21:48:21) [0x80fa028] [rhythmdb_property_model_finalize] rhythmdb-property-model.c:478: finalizing property model 0x8714520
(21:48:21) [0x80fa028] [rb_source_finalize] rb-source.c:331: Finalizing source 0x8750820: 'Radio'
(21:48:21) [0x80fa028] [rb_source_finalize] rb-source.c:334: Unreffing model 0x847da20 count: 1
(21:48:21) [0x80fa028] [rhythmdb_query_model_dispose] rhythmdb-query-model.c:633: disposing query model 0x847da20
(21:48:21) [0x80fa028] [rhythmdb_query_model_finalize] rhythmdb-query-model.c:681: finalizing query model 0x847da20
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Internet Radio
(21:48:21) [0x80fa028] [rb_iradio_plugin_finalize] rb-iradio-plugin.c:82: RBIRadioPlugin finalising
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Audio CD Player
(21:48:21) [0x80fa028] [rb_audiocd_plugin_finalize] rb-audiocd-plugin.c:133: RBAudioCdPlugin finalising
(21:48:21) [0x80fa028] [impl_deactivate] rb-cd-recorder-plugin.c:371: RBCdRecorderPlugin deactivating
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Audio CD Recorder
(21:48:21) [0x80fa028] [rb_cd_recorder_plugin_finalize] rb-cd-recorder-plugin.c:121: RBCdRecorderPlugin finalizing
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Portable Players
(21:48:21) [0x80fa028] [rb_generic_player_plugin_finalize] rb-generic-player-plugin.c:110: RBGenericPlayerPlugin finalising
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Song Lyrics
(21:48:21) [0x80fa028] [rb_python_object_finalize] rb-python-plugin.c:202: Finalizing python plugin instance
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Media Player Keys
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Power Manager
(21:48:21) [0x80fa028] [rb_plugin_info_free] rb-plugins-engine.c:362: Unref plugin Cover art
(21:48:21) [0x80fa028] [rb_python_object_finalize] rb-python-plugin.c:202: Finalizing python plugin instance
(21:48:21) [0x80fa028] [rb_podcast_source_shutdown] rb-podcast-source.c:1783: podcast source shutdown
(21:48:21) [0x80fa028] [rb_shell_sync_state] rb-shell.c:806: saving playlists
(21:48:21) [0x80fa028] [rb_shell_sync_state] rb-shell.c:810: saving db
(21:48:21) [0x80fa028] [rhythmdb_save] rhythmdb.c:2581: saving the rhythmdb and blocking
(21:48:21) [0x80fa028] [rhythmdb_save_async] rhythmdb.c:2565: saving the rhythmdb in the background
(21:48:21) [0x80fa028] [rhythmdb_read_enter] rhythmdb.c:907: counter: 1
(21:48:21) [0x841aae0] [rhythmdb_save_thread_main] rhythmdb.c:2516: entering save thread
(21:48:21) [0x841aae0] [rhythmdb_save_thread_main] rhythmdb.c:2531: saving rhythmdb
(21:48:21) [0x841aae0] [save_entry_type] rhythmdb-tree.c:1054: saving entries of type song
7dfa28f4-e248-2387-7d4d4f88-71ee9a16 is dumped
(21:48:21) [0x841aae0] [save_entry_type] rhythmdb-tree.c:1054: saving entries of type ignore
(21:48:21) [0x841aae0] [save_entry_type] rhythmdb-tree.c:1054: saving entries of type podcast-feed
(21:48:21) [0x841aae0] [save_entry_type] rhythmdb-tree.c:1054: saving entries of type iradio
(21:48:21) [0x841aae0] [save_entry_type] rhythmdb-tree.c:1054: saving entries of type podcast-post


Other information:
let me know if a stack trace is really needed to fix this, on this distro is not so simple ... but please consider I can reproduce the error on a regular basis following the 3 steps in the header
thanks :+)