0) This is a gtkhtml related crash. Filed here, because I seem to remember only evolution developers care.

1) Crash happens when trying to close evolution after trying to accept a meeting without an organizer. (Complicating factor: that meeting request was sent as a TNEF Encoded attachment. Maybe my current attempts to make the TNEF Attachment decoder plugin work are to blame.)

2) Trying to accept that meeting will trigger this "Evolution Warning":
"This message cannot be sent because you have not specified any recipients

Please enter a valid email address in the To: field. You can search for email addresses by clicking on the To: button next to the entry box."

3) After closing evolution a lot of messages appeared to have been printed to stdout/stderr:
** Message: Response 1

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_radio_action_get_current_value: assertion `GTK_IS_RADIO_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_get_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_set_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_get_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_set_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_get_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_set_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_get_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_set_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_table_get_account: assertion `E_IS_COMPOSER_HEADER_TABLE (table)' failed

(evolution:8060): e-attachment-view.c-CRITICAL **: e_attachment_view_get_store: assertion `E_IS_ATTACHMENT_VIEW (view)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_table_get_subject: assertion `E_IS_COMPOSER_HEADER_TABLE (table)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_table_get_account: assertion `E_IS_COMPOSER_HEADER_TABLE (table)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_table_get_reply_to: assertion `E_IS_COMPOSER_HEADER_TABLE (table)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_table_get_header: assertion `E_IS_COMPOSER_HEADER_TABLE (table)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_get_visible: assertion `E_IS_COMPOSER_HEADER (header)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_table_get_header: assertion `E_IS_COMPOSER_HEADER_TABLE (table)' failed

(evolution:8060): composer-CRITICAL **: e_composer_header_get_visible: assertion `E_IS_COMPOSER_HEADER (header)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_get_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed

(evolution:8060): Gtk-CRITICAL **: gtk_ui_manager_get_action_groups: assertion `GTK_IS_UI_MANAGER (self)' failed

(evolution:8060): gtkhtml-editor-CRITICAL **: gtkhtml_editor_get_action: assertion `action != NULL' failed

(evolution:8060): Gtk-CRITICAL **: gtk_toggle_action_get_active: assertion `GTK_IS_TOGGLE_ACTION (action)' failed
Segmentation fault (core dumped)

4) backtrace:

+ Trace 219071

5) My suggestions:
a) Maybe gtk_html_command() could be changed like this:
--- gtkhtml/gtkhtml.c.crash	2009-08-09 16:29:34.000000000 +0200
+++ gtkhtml/gtkhtml.c	2009-11-13 22:48:47.000000000 +0100
@@ -6005,7 +6005,7 @@ gtk_html_command (GtkHTML *html, const g
 	class = G_ENUM_CLASS (g_type_class_ref (GTK_TYPE_HTML_COMMAND));
 	val = g_enum_get_value_by_nick (class, command_name);
 	g_type_class_unref (class);
-	if (val) {
+	if (html && val) {
 		if (command (html, val->value)) {
 			if (html->priv->update_styles)
 				gtk_html_update_styles (html);

b) Maybe gtkhtml_editor_run_command() could also be changed:
--- ./components/editor/gtkhtml-editor.c.crash	2009-07-03 06:46:11.000000000 +0200
+++ ./components/editor/gtkhtml-editor.c	2009-11-13 22:52:49.000000000 +0100
@@ -1539,6 +1539,8 @@ gtkhtml_editor_run_command (GtkhtmlEdito
 	g_return_val_if_fail (command != NULL, FALSE);
 
 	html = gtkhtml_editor_get_html (editor);
+	if (!html)
+		return FALSE;
 
 	return gtk_html_command (html, command);
 }

Neither suggestion is tested (I haven't even tried to compile them). I wouldn't mind testing these, writing real patches and maybe even committing some patches. So feel free to ask me to test, write and/or commit.