Prevent GTK assertions due to scrolled window not being initialised.

Seems to be a gtk bug which we need to work around. The assertions
don't actually seem to cause any harm (they just print a bunch of
"Gtk-CRITICAL **: IA__gtk_range_get_adjustment: assertion `GTK_IS_RANGE (range)' failed"
but probably best to avoid them.

Change-Id: I5d1bb20bd5c0569c6d023a6148123208a15b9de2