Null check on NavigationBarView in NavigationBarFragment

While sysui is starting, it's possible to get disable callbacks before
the NavigationBarView has had a chance to get created. In this case just
don't pass the message, similar to Disable1 callbacks

Change-Id: Ic89ca281579ff9720c356f79102d1b576670740a
Fixes: 118357487
Test: no exceptions during system boot after wiping