Flutter StreamBuilder<DocumentSnapshot> not passing value into child StreamBuilder - flutter

Question

I am trying to get a specific icon to appear based on whether the user has taken the survey or not.
At the moment, I am using StreamBuilder<DocumentSnapshot> to listen for a given value in a document, which returns the survey name. I then want to use the survey name in the next StreamBuilder, which will look in a given collection (which is made up of the survey name followed by _entrants - so, for example, Survey_entrants) for a completed survey document, which will have the title of the user's unique id (named userid).
The problem I have now is that whilst surveyName does return the name of the survey put in Cloud Firestore, and updates it when I change the value (I can see this by the commented-out return new Text('$surveyName'); command).
However, it does not seem to be passing that value into the next StreamBuilder - regardless of what I put in as the survey name, I get the check icon showing, suggesting (snapshot1.hasData) - even when that document does not exist.
I know the surveyName variable is working, but if I do snapshot1.toString() I get the error Snapshot<DocumentSnapshot>(ConnectionState.active, Instance of 'DocumentSnapshot', null). This must count has having data, hence showing the survey being taken. How do I correct this?
My code:
Positioned(
right: 30,
top: 20,
child: StreamBuilder<DocumentSnapshot>(
stream: Firestore.instance
.collection('Controller')
.document('Current Survey')
.snapshots(),
builder: (BuildContext context,
AsyncSnapshot<DocumentSnapshot> snapshot) {
if (!snapshot.hasData) {
return CircularProgressIndicator();
} else {
var sn = snapshot.data;
surveyName = sn["cs"];
// return new Text('$surveyName');
return StreamBuilder(
stream: Firestore.instance
.collection('$surveyName' + '_entrants')
.document(userid)
.snapshots(),
builder: (BuildContext context, snapshot1) {
if (!snapshot1.hasData) {
return Icon(
Foundation.burst_new,
size: 48,
color: Color(0xff303841),
);
} else if (snapshot1.hasData) {
return Icon(
Foundation.check,
size: 48,
color: Color(0xff303841),
);
} else {
return Icon(
MaterialIcons.error_outline,
size: 48,
color: Color(0xff303841),
);
}
});
}
})),