RemoteInputView: Add null check in WindowInsetsAnimation.Callback

We're seeing a rare but consistent crash here, so add a null check and
log an error. Ideally we'll get a repro case and find a root cause, but
this will mitigate the crash for now.

Bug: 219371276
Test: none, since we have no repro case
Change-Id: Ia8152f854c8d4fb8a85e221ffa1c781c2923cf40