How to fix JSON value '' of type NSNull cannot be converted to NSString in iOS react-native-config and AsyncStorage?

I'm having problems with react native config and AsyncStorage, where null data from .env is not readable on iOS,

below is my code:

  _authAsync = async () => {
    const userToken = await syncStorageA.getItem(Config.ACCESS_TOKEN)
    this.props.navigation.navigate(userToken ? 'App' : 'Auth')
  }

Please help.

I'm having this problem only on the iOS platform on Android is working fine.