Questions on Flutter DropdownButton Value - flutter

Question

Hello I'm new to flutter and I'm trying to create multiple DropdownButtons based on different documents in my Firebase backend. Since there may be multiple DropdownButton (different documents have different number of dropdowns to be created), I wanted to maintain there values with List<String> formInput (formInput[0] maintains the first DropdownButton value etc).
But I keep encountering this error, and came to the conclusion that it is was the formInput[i] fault.
So my question is why can't I use formInput[i] as a value for a Dropdownbutton? Is formInput[i] not considered one item? And is there a better solution for maintaining unknown number of values?
Many Thanks
Code:
class _SubmissionFormState extends State<SubmissionForm> {
List<String> formInput;
var test;
#override
Widget build(BuildContext context) {
Size size = MediaQuery.of(context).size;
return FutureBuilder(
future: getStringFromPref('site'),
builder: (context, snapshot) {
if (snapshot.hasData) {
String site = snapshot.data;
return StreamBuilder(
stream: FirebaseFirestore.instance
.collection('form')
.where('name', isEqualTo: site)
.snapshots(),
builder: (context, snapshot) {
if (snapshot.hasData) {
int inputSize = snapshot.data.documents.length;
List<Widget> listOfWidget = [];
formInput = new List<String>.filled(inputSize, '-');
print(formInput.length);
for (int i = 0; i < inputSize; i++) {
DocumentSnapshot snap = snapshot.data.documents[i];
List<DropdownMenuItem> options = [];
// adding drop down menu items
for (int j = 0; j < snap['formVal'].length; j++) {
String formVal = snap['formVal'][j];
options.add(DropdownMenuItem(
child: Text(
formVal,
),
value: formVal,
));
print('[inner loop] for loop $i ' + snap['formVal'][j]);
}
// the list of DropdownButtons
listOfWidget.add(DropdownButton(
value: formInput[i],
items: options,
hint: Text(
"-",
),
onChanged: (value) {
setState(() {
formInput[i] = value;
});
},
));
// listOfWidget.add(Text(snap['formTxt']));
}
return Column(
children: listOfWidget,
);
} else {
return CircularProgressIndicator();
}
});
} else {
return CircularProgressIndicator();
}
},
);
}
}
Error:
I/flutter (30860): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (30860): The following assertion was thrown building StreamBuilder<QuerySnapshot>(dirty, state:
I/flutter (30860): _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>>#60d57):
I/flutter (30860): There should be exactly one item with [DropdownButton]'s value: -.
I/flutter (30860): Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
I/flutter (30860): 'package:flutter/src/material/dropdown.dart':
I/flutter (30860): Failed assertion: line 839 pos 15: 'items == null || items.isEmpty || value == null ||
I/flutter (30860): items.where((DropdownMenuItem<T> item) {
I/flutter (30860): return item.value == value;
I/flutter (30860): }).length == 1'
I/flutter (30860):