Flutter property values of a Map all treated as strings - list

Question

I have a list like this a = [{'one': 'one', 'two': null, 'three': [{'four': 'four'}]}]
I send it to a function to use it in a post request which in the body should receive a Map, so what I did was this to a[0], the problem is that I get this error The getter 'length' was called on null
I start to review and it treats all the property values as if they were Strings, even the nested list 'three': [{'four': 'four'}], I have tried to send the post in this way http.post (url, body: (recurrence [0] as Map)) but it has not worked, it always gives me the same error, even if in the body I put the properties by hand in the body: {'new property': a [0] [' tres']}, how should one act to solve this problem? Thank you very much for your help
Code:
void _ordersGet() async {
await http.get(url).then((value) {
setState(() {
orders = jsonDecode(value.body);
}
}
orders is sent to a new widget: orderList(orders)
orderList is a listView
ListView.builder(
shrinkWrap: true,
primary: false,
itemCount: orders.length,
itemBuilder: (orders, index) {
return return Card(
elevation: 5,
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Text(orders[index]['facts']),
SizedBox(
width: 4,
),
Text('Cantidad : '),
Text(orders[index]['ITEMS'][0]['jeans']),
SizedBox(
width: 4,
),
IconButton(
onPressed: () => _reorderData(context, orders[index]),
icon: Icon(
Icons.replay_outlined,
color: Theme.of(context).accentColor,
)),
],
),
);
},
);
_reorderData is a function that make a get request, the info in shipped to ReorderModal
ReorderModal it only shows the information and has a button
void _reorderData(BuildContext ctx, order) async {
var data;
var url = 'serverAddress/${order['facts']}';
await http.get(url).then((value) {
data = jsonDecode(value.body);
data[0]['CORPORATION'] = order['corporation'];
showModalBottomSheet(
context: ctx,
builder: (_) {
return ReorderModal(data);
});
}).catchError((onError) {});
}
class ReorderModal extends StatelessWidget {
final List data;
ReorderModal(this.data);
void orderSend(orderInfo) async {
var url = 'serverAddress';
await http.post(url, body: orderInfo[0]).then((value) {
print(jsonDecode(value.body));
}).catchError((onError) {
print(onError);
});
}
#override
Widget build(BuildContext context) {
return Padding(
padding: const EdgeInsets.all(10),
child: Column(children: <Widget>[
Row(
children: <Widget>[
Expanded(
child: Column(
children: [
ElevatedButton(
onPressed: () {
orderSend(data);
//print(data);
},
child: Text('ONE Click'))
]),
);
}
}
when i press the ONE Click button execute the function orderSend, orderSend make a post request and the problem described above
This is the simplified code, I know it must be something very simple, but it is giving me a lot of work to solve