@Nullable
  @Override
  @CallSuper
  public List<String> validate(@NonNull JobTrigger trigger) {
    if (trigger != Trigger.NOW
        && !(trigger instanceof JobTrigger.ExecutionWindowTrigger)
        && !(trigger instanceof JobTrigger.ContentUriTrigger)) {
      return getMutableSingletonList("Unknown trigger provided");
    }

    return null;
  }
Attempts to validate the provided Trigger. If valid, null is returned. Otherwise a list of
errors will be returned.

<p>Note that a Trigger that passes validation here is not necessarily valid in all permutations
of a JobParameters. For example, an Immediate is never valid for a recurring job.
jobdispatcher/src/main/java/com/firebase/jobdispatcher/DefaultJobValidator.java