You should not be using yield statements if you are not using tasks. Here is a sample script that displays a dialog without using tasks. Use the spacebar to toggle the visibility of the dialog.
Code:
import viz
import vizdlg
viz.go()
#Create empty dialog
dlg = vizdlg.Dialog(title='Dialog title')
#Link dialog to center of window
viz.link(viz.CenterCenter,dlg)
#Toggle dialog visibility when spacebar is pressed
vizact.onkeydown(' ',dlg.visible,viz.TOGGLE)
viz.add('gallery.ive')