Concurrent modification during iteration: Instance(length:3) of '_GrowableList' - flutter

Question

I am new in flutter and i want to do pagination. I am using one package which name is pull to refresh. All work are doing well but when I reach last data of list then current page is increment with +1 and api call of page=2. in my code its open new page, i want to append with current listview. When i write code for append its throw error "Concurrent modification during iteration: Instance(length:3) of '_GrowableList'."(my english is not good but I hope you are understood).
Following are my code.
int currentPage = 1;
bool isRefersh = false;
Future<UserPost> getUserPost() async {
var url =
"https://myApiLink/getUserPost.php?page=$currentPage";
var response = await http.get(Uri.parse(url));
var jsondata = jsonDecode(response.body.toString());
var _apiData = UserPost.fromJson(jsondata);
if (response.statusCode == 200) {
if(isRefersh == true){
setState((){
isRefersh = false;
});
refreshController.refreshCompleted();
return UserPost.fromJson(jsondata);
}
else{
print(_apiData.hasNextPage.toString());
if(_apiData.hasNextPage == 0){
refreshController.loadNoData();
}else{
refreshController.loadComplete();
}
return UserPost.fromJson(jsondata);
}
} else {
return UserPost.fromJson(jsondata);
}
}
My list view code:
FutureBuilder<UserPost>(
future: getUserPost(),
builder: (context, snapshot) {
if (snapshot.hasData) {
return SmartRefresher(
controller: refreshController,
enablePullUp: true,
child: ListView.builder(
//close keyboard while scroll
keyboardDismissBehavior:
ScrollViewKeyboardDismissBehavior.onDrag,
//itemCount: isFilter? ComparisonText.length : snapshot.data!.data!.length,
itemCount: snapshot.data!.data!.length,
itemBuilder: (context, i) {
var postList = snapshot.data!.data![i];
String tempSearch =
postList.azakhanaName.toString();
)}
onRefersh for refreshing and onLoading for pagination.
onRefresh: () async{
await Future.delayed(Duration(milliseconds: 1000));
setState(() {
isRefersh = true;
currentPage = 1;
});
},
onLoading: () async {
if(snapshot.data!.hasNextPage == 0){
refreshController.loadNoData();
}else{
setState(() {
currentPage++;
snapshot.data!.data!.addAll(snapshot.data!.data!);
});
await Future.delayed(Duration(milliseconds: 1000));
refreshController.loadComplete();
}
},
I am trying to append listview with new data like this and and error also throw in this code:
snapshot.data!.data!.addAll(snapshot.data!.data!);