Error in loading data from cloud firestore - firebase

Question

body: StreamBuilder(
stream: Firestore.instance.collection('students').document(_userId).snapshots(),
builder: (context,snapshot) {
return Container(
child: Column(
mainAxisAlignment: MainAxisAlignment.start,
children: <Widget>[
SizedBox(
height: 20.0,
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Padding(
padding: const EdgeInsets.only(left:20.0),
child: Align(
alignment: Alignment.centerRight,
child: CircleAvatar(
radius: 100,
backgroundColor: Colors.white,
child: ClipOval(
child: new SizedBox(
width: 180.0,
height: 180.0,
child: (_image!=null)?Image.file(
_image,
fit: BoxFit.fill,
):
Image.network(""),
),
),
),
),
),
Padding(
padding: EdgeInsets.only(top: 60.0),
child: IconButton(
icon: Icon(
Icons.camera,
size: 30.0,
),
onPressed: () {
getImage();
},
),
),
],
),
SizedBox(
height: 20.0,
),
Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
Padding(
padding: const EdgeInsets.only(left:40.0),
child: Align(
alignment: Alignment.center,
child: Container(
child: Column(
children: <Widget>[
Align(
alignment: Alignment.center,
child: Text('Student Name',
style: TextStyle(
color: Colors.white, fontSize: 20.0)),
),
Align(
alignment: Alignment.center,
child: Text(snapshot.data['first_name'],
style: TextStyle(
color: Colors.black,
fontSize: 18.0,
)),
),
],
),
),
),
),
i want to display student name from cloud firestore but it gives an error upon first loading but after hotreload the app works correctly.
here is my student id retrieving function
FirebaseAuth.instance.currentUser().then((user) {
_userId = user.uid;
});
i am using this id for retrieving a particular student details but it give error upon first loading the error are enter image description here
The database snap are enter image description here