Fixing NPE for AppWidgetManagerServiceImpl.

In ag/26226975 widgets were blocked for apps
running in hidden profiles.
However, this leads to a NPE as shown here:
https://android-build.corp.google.com/test_investigate/?invocationId=I97100010248393459&testResultId=TR38829184850093013&tab=artifacts

This change adds a check for NPE.

Bug: 287975131
Test: atest FrameworksServicesTests
Flag: ACONFIG android.multiuser.disable_private_space_items_on_home
DEVELOPMENT

Change-Id: I40818c2896e21d53f9ce72d81cf81926f7ac2260