webadmin: Guard against premature CommonModel nullification

This patch ensures that CommonModel instance gets nullified via
deferred command, only after current UiCommon-related processing
is over.

In case some SyncSearch response callback still executes after
that, "getModel() != null" condition in DataBoundTabModelProvider
should guard against potential NullPointerExceptions.

This patch also removes "CommonModel != null" assertions from
UiCommonModelResolver, since it's possible for CommonModel
reference to be null in some edge-case scenarios (i.e. SyncSearch
response callback being executed after user signs out).

Change-Id: I2bf55e75d97006d230bf503fac026722321435d0
Bug-Url: https://bugzilla.redhat.com/924670
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>