Steps to reproduce:

- Search for a new Podcast (either using an URL or a search term that produces only one result)
- Click Subscribe
- Click Subscribe a second time
- Crash in subscribe_selected_feed()

Error message:

(rhythmbox:13626): Gtk-CRITICAL **: gtk_list_store_get_value: assertion `iter_is_valid (iter, list_store)' failed
(rhythmbox:13626): GLib-GObject-WARNING **: gtype.c:4204: type id `0' is invalid
(rhythmbox:13626): GLib-GObject-WARNING **: can't peek value table for type `<invalid>' which is not currently referenced

Backtrace:

+ Trace 231725

The fault is obviously that the subscibe butten doesn't get disabled after the first click.
It should get disabled in feed_selection_changed_cb() which is invoked by gtk_list_store_remove() in
subscribe_clicked_cb()
[...]
	dialog->priv->clearing = TRUE;
	gtk_list_store_remove (GTK_LIST_STORE (dialog->priv->feed_model), &dialog->priv->selected_feed);
	dialog->priv->clearing = FALSE;

in the callback:
feed_selection_changed_cb (GtkTreeSelection *selection, RBPodcastAddDialog *dialog)
{
	GtkTreeModel *model;

	if (dialog->priv->clearing)
		return;

	dialog->priv->have_selection =
		gtk_tree_selection_get_selected (selection, &model, &dialog->priv->selected_feed);
	gtk_widget_set_sensitive (dialog->priv->subscribe_button, dialog->priv->have_selection);

So the CB returns directly without doing the necessary call to gtk_widget_set_sensitive().
In case the feed list has several entries the callback will be invoked again after 
gtk_tree_selection_unselect_all() in subscribe_clicked_cb() and the subscribe button will be disabled.

A simple fix is to always call gtk_widget_set_sensitive() in feed_selection_changed_cb().