webadmin: A NPE check in SetupNetworksModel:onEdit

There's a possible branch of statement, that if executed
a null value will be dereferenced. this patch verifies that the model
editPopup is not null before dereferenced.

Change-Id: I165a3f298bc306d65cefceef3a8dfd24dc366774
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>