Fix unmount criticals

[vjr]

Fix the following during unmount:

(io.elementary.files:371186): GLib-GObject-CRITICAL **: 23:48:11.625: invalid (NULL) pointer instance

(io.elementary.files:371186): GLib-GObject-CRITICAL **: 23:48:11.625: g_signal_emit_by_name: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

(io.elementary.files:371186): Gtk-CRITICAL **: 23:48:11.625: gtk_widget_destroy: assertion 'GTK_IS_WIDGET (widget)' failed

[jeremypw]

You don't really need an early return here, you can save a line by testing for dialog != null

Addressed in 39edbd5

[jeremypw]

Thanks!