Return null when setting can't be changed.

If a Slice cannot be changed, return null instead of
an error slice.

Bug: 80155832
Test: robotests
Change-Id: I843fee76cf19d49cc994062059cb231f222120b2
Merged-In: Ib94136c449c6d9c1911f89833bba62fd2263daa4
5 files changed