Fix NullPointerException on addErrorCode when input is Invalid error code am: ad90e1bfe1 am: c2cdb2c764

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/NetworkStack/+/11987398

Change-Id: I56d1e71a1ce100a5306f5a53abe25d194c7f869b