The getter 'length' was called on null in post request - flutter

Question

I'm trying to make a post request to an API, and I couldn't get why this is happening.
This is my code:
print('THIS IS PRINTING');
var response = await http.post(url, body: body, headers: header);
print('This not printing (throwing error before it prints)');
This is the error that I'm getting:
I/flutter ( 2417): NoSuchMethodError: The getter 'length' was called on null.
I/flutter ( 2417): Receiver: null
I/flutter ( 2417): Tried calling: length
I'm able to make the request using PostMan, and giving the same fields and values, I get this error on dart. I'm also able to make other get requests without any issues.
Full code:
void getFavorites() async {
var url = "https://www.my-url-edited.com/favorite";
var header = {
"Content-Type": "application/x-www-form-urlencoded",
"token": "my token ---- edited",
};
var body = {
"car_id": "1",
"boat_id": null,
"habitation_id": null,
"product_id": null,
};
try {
print('print before post request --- working');
// var response = await http.post(url, body: body, headers: header);
var response = await http.post(url, body: body, headers: header);
print('This not printing (throwing error before it prints)');
print(response.body);
// var data = json.decode(response.body);
} catch (e) {
print(e);
throw Exception('Not connected to network');
}
}

Answer 1

you header should be ,
headers: {'Content-type': 'application/json','Accept': 'application/json'}

Answer 2

Try
var header = {
"Content-Type": "application/json",
"token": "my token ---- edited",
};
var body = jsonEncode({
"car_id": "1",
});