电子邮件启动器中的NULL值

Question

我从firestore中读取数据，并将其用于电子邮件的url启动器。但是我在提示打开电子邮件时得到的值是NULL。但是它不会提示任何错误。

我列出的将数据设置为var的代码。

one_to_many

用于从Firestore获取数据以获取用户详细信息和发件人电子邮件地址的代码

var userName, userAdd, userAge, userResult, uniEmail;

void emailDetail() async{
  Widget build(BuildContext context) {
      return new StreamBuilder(
          stream: UniQuery().getUserDetail(),
          builder: (BuildContext context, snapshot) {
            if (!snapshot.hasData)
              return new Text('Loading..... Please wait');

            var userDocument = snapshot.data;
             userName = userDocument['name'];
             userAge = userDocument['age'];
             userAdd = userDocument['address'];
             userResult= userDocument['result'];

             return new Text('Success read user data');
          }
      );
  }
}

void emailAddress() async{
  Widget build(BuildContext context) {
      return new StreamBuilder(
          stream: UniQuery().getUniBasedSelectedUni(),
          builder: (BuildContext context, snapshot) {
            if (!snapshot.hasData)
              return new Text('Loading..... Please wait');

            var userDocument = snapshot.data;
            uniEmail = userDocument['email'];

            return new Text('Success read university email');
          }
      );
  }
}

void sendEmail() async {
  emailDetail();
  emailAddress();
  var url = 'mailto:$uniEmail?subject= University Application'
      +'&body= Name: $userName \nAge: $userAge \nAddress: $userAdd'
      +'\nResult: $userResult';
  if (await canLaunch(url)) {
    await launch(url);
  }
  else {
    throw 'Could not launch $url';
  }
} 

我在这里称它为sendEmail

   Future<String> getCurrentUser() async{
    final FirebaseUser user = await FirebaseAuth.instance.currentUser();
    final String uID = user.uid.toString();
    return uID;
  }

  setUserDetail() async{
    userID = await getCurrentUser();
  }

  getUserDetail(){
    setUserDetail();
    return dbReference.collection('User')
        .document(userID)
        .snapshots();
  }

  getUniBasedSelectedUni(){
    return dbReference.collection('University')
        .where('name', isEqualTo: selectedUni)
        .snapshots();
  }