frontend: Fix NPEx in new vm dialog when the DC is unitialized

This patch adds a null check that avoids NPEx after trying to add a new
VM when there is no initialized DC.

Change-Id: I04349ddfe9ce164d85c818c439c8c4f89ae01019
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>