Line after await http response in Flutter app does not execute - flutter

Question

I want to fetch a result from Flask API after sending http.MultipartFile request to server.
like this:
Future<String> upload(List<int> filename, String url) async {
//filename : binary conversion of string
//String url : api address
print("Inside Upload future!");
var request = http.MultipartRequest('POST', Uri.parse(url));
request.files.add(
http.MultipartFile.fromBytes('file', filename, filename: 'files.txt'));
print("Sending Request!");
http.Response response = await http.Response.fromStream(await request.send());
print("request sent! now returning");
var rtr = response.body;
return rtr;
}
But the problem is it does not return, and output after execution is The print after await is not executed why??:
Inside Upload future!
Sending Request!
I am sending a String to Flask API like:
It works correctly, it is receiving a string and then replying with the same string with some modifications.
#app.route('/test_api', methods=['GET', 'POST'])
def test_api():
uploaded_file = request.files['file']
file_content = uploaded_file.read().splitlines()
uploaded_file.seek(0)
file_pretty = uploaded_file.read()
a = runkey(file_pretty)
//takes string agrument and returns manipulated string
uploaded_file.seek(0)
filename = secure_filename(uploaded_file.filename)
resp = make_response(a)
resp.headers['Content-Type'] = 'text/plain;charset=UTF-8'
n = filename
resp.headers['Content-Disposition'] = 'attachment;filename='+'n'
return resp ```

Answer 1

I Solved this, By Adding
resp.headers['Access-Control-Allow-Origin'] = '*'
//before returning response to client
to the FlaskAPI response.
actually, I thought there's an error in Dart somewhere. But it was hiding in Chrome debugging, Since I am using Flutter Web.
The error is of Cross-Origin not allowed due to access control.