Dropdown list not showing selected value - flutter

Question

I have a dropdown list that gets its value from firebase and when I'm trying to pick a value, it shows me an error.
There should be exactly one item with [DropdownButton]'s value: vff.
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 850 pos 15: 'items == null || items.isEmpty || value == null ||
items.where((DropdownMenuItem<T> item) {
return item.value == value;
}).length == 1'
here's the source code, I was adding values to dropdown list by using loop, and giving value by name of query
return StreamBuilder<List<CoursesRecord>>(
stream: queryCourseRecord(
queryBuilder: (courseRecord) => courseRecord.orderBy(/*widget.orderBy*/'${widget.orderBy}', descending: true), limit: 5,
),
builder: (context, snapshot) {
// Customize what your widget looks like when it's loading.
if (!snapshot.hasData) {
return Center(
child: SizedBox(
width: 50,
height: 50,
child: CircularProgressIndicator(
color: FlutterFlowTheme.primaryColor,
),
),
);
}
List<CoursesRecord> listViewCourseRecordList = snapshot.data;
for (int i = 0; i < 2; i++) {
String value = listViewCourseRecordList[i].name;
courseItems.add(
DropdownMenuItem(
child: Container(
width: 316 ,
child: CourseBoxName(coursesRecord: listViewCourseRecordList[i],)),
value: value,
),
);
}
// Customize what your widget looks like with no query results.
if (snapshot.data.isEmpty) {
return Container(
height: 100,
child: Center(
child: Text('No results.'),
),
);
}
return Container(
width: MediaQuery.of(context).size.width,
height: 100,
child: DropdownButtonHideUnderline(
child: ButtonTheme(
alignedDropdown: true,
child: DropdownButton(
value: selected,
isDense: true,
items: courseItems,
hint: Text(
"Выберите курс",
),
onChanged: (newValue) {
setState(() {
selected = newValue;
widget.courseSelected(newValue);
});
},
isExpanded: false,
),
),
),
);
},
);
So what can I do to fix this problem, I have stuck with it already for several hours T_T

Answer 1

You got null value so for check conditions like droupdownvalue ?? "xyz"
So you can see XYZ when you got a null value.

Answer 2

it looks like you keep adding items to a list with every build without clearing it
on first build you add 1,2,3 and the list contains [1,2,3]
on second build you add the same items even though they didn't change, and the list contains [1,2,3,1,2,3] which clearly contains duplicates
remove courseItems field and the for loop, then tweak the button definition like following:
DropdownButton(
value: selected,
isDense: true,
items: [
for (int i = 0; i < 2; i++)
DropdownMenuItem(
child: Container(
width: 316,
child: CourseBoxName(
coursesRecord: listViewCourseRecordList[i],
)),
value: value,
)
],
hint: Text(
"Выберите курс",
),
onChanged: (newValue) {
setState(() {
selected = newValue;
widget.courseSelected(newValue);
});
},
isExpanded: false,
)