Fix invalid cast warning

[Scott Oonk]

This patch fixes an invalid cast warning when selecting an account in
the accounts tab of the options dialog.

(gnucash:32061): GLib-GObject-WARNING **: invalid cast from
`GtkTreeSelection' to `GtkWidget'

(gnucash:32061): Gtk-CRITICAL **: gtk_widget_get_parent: assertion
`GTK_IS_WIDGET (widget)' failed
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