Patch for 85399 - nautilus crashes on closing the last windows after preferences had been opened

[Damon Chaplin <damon ximian com>]

I've added a patch here:

  http://bugzilla.gnome.org/show_bug.cgi?id=85399

It was missing an argument to g_signal_emit, so it was failing when
trying to collect the value.

Also, when the dialog is destroyed it clears the selection, so the
selected theme can be NULL. I added code to ignore that case.

Damon