public @Nullable String getStringExtra(String key) {
    if (mExtraData == null) {
      return null;
    }
    // concurrenthashmap will throw NPE when key parameter is null
    if (key == null) {
      return null;
    }
    Object val = mExtraData.get(key);
    return val == null ? null : String.valueOf(val);
  }
Get the top level context chain, which can be the current context chain itself.
fbcore/src/main/java/com/facebook/common/callercontext/ContextChain.java