Add null check for mCallback in AutoFillUI

Bug: b/349731339
Test: atest CtsAutofillServiceTestCases
Flag: EXEMPT refactor
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ffbe20e7d3c2322fdf0c536a3c16c22e90d72ca6)
Merged-In: I7d3f8fe399cb4e83f5f5fb80e27d232dad3b5b74
Change-Id: I7d3f8fe399cb4e83f5f5fb80e27d232dad3b5b74
1 file changed