Fix NPE when coming back to settings homepage.

When users navigate back to the homepage, if there is a slice being
displayed, slice callback will be triggered. It will then check if this
slice has any error hint, where NPE would occur if the slice turns out
to be null. Hence, add a null check before checking slice hint.

Fixes: 148158780
Test: rebuild and navigate back to the homepage.
Change-Id: I2b04dc0d64a223cc85f6af55b2ae2dbc5f3e1f40