Fix Chrome crash at "->GetFieldById"

This CL fixes a crash at calling "->GetFieldById" where the caller might
be nullptr.

Bug: 331563354, 331706754
Change-Id: I34cfe17ef871dbd5e3366726d5cb50a264ef532f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5403406


Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Timofey Chudakov <tchudakov@google.com>
Reviewed-by: Olivia Saul <jsaul@google.com>
Commit-Queue: Qihui Zhao <qihuizhao@google.com>
Cr-Commit-Position: refs/heads/main@{#1279961}