POST requests to Flask from Unity result in `null` values - unity3d

Question

After getting this demo server working I am able return GET requests from it to Unity, but when I would try to send data from Unity to the local server using POST requests it would only show null values added into the server. This is the code I was using in Unity:
IEnumerator Upload()
{
WWWForm form = new WWWForm();
form.AddField("charge","+4/3");
form.AddField("name", "doubletop");
using (UnityWebRequest www = UnityWebRequest.Post("http://localhost:5000/quarks/", form))
{
yield return www.SendWebRequest();
if (www.isNetworkError || www.isHttpError)
{
Debug.Log(www.error);
}
else
{
Debug.Log("Form upload complete!");
}
}
}
I would get "Form upload complete!" in the console, and GET requests would work, but those null values kept coming.

Answer 1

I modified my Upload() method to the PostRequest() in this example, and now it works!
Here's the full code:
using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Networking;
public class HTTP : MonoBehaviour
{
void Start()
{
// A correct website page.
StartCoroutine(GetRequest("localhost:5000/quarks"));
PostData();
StartCoroutine(GetRequest("localhost:5000/quarks"));
// A non-existing page.
//StartCoroutine(GetRequest("https://error.html"));
}
IEnumerator GetRequest(string uri)
{
using (UnityWebRequest webRequest = UnityWebRequest.Get(uri))
{
// Request and wait for the desired page.
yield return webRequest.SendWebRequest();
string[] pages = uri.Split('/');
int page = pages.Length - 1;
if (webRequest.isNetworkError)
{
Debug.Log(pages[page] + ": Error: " + webRequest.error);
}
else
{
Debug.Log(pages[page] + ":\nReceived: " + webRequest.downloadHandler.text);
}
}
}
[Serializable]
public class Quark
{
public string name;
public string charge;
}
public void PostData()
{
Quark gamer = new Quark();
gamer.name = "doublebottom";
gamer.charge = "4/3";
string json = JsonUtility.ToJson(gamer);
StartCoroutine(PostRequest("http://localhost:5000/quarks", json));
}
IEnumerator PostRequest(string url, string json)
{
var uwr = new UnityWebRequest(url, "POST");
byte[] jsonToSend = new System.Text.UTF8Encoding().GetBytes(json);
uwr.uploadHandler = (UploadHandler)new UploadHandlerRaw(jsonToSend);
uwr.downloadHandler = (DownloadHandler)new DownloadHandlerBuffer();
uwr.SetRequestHeader("Content-Type", "application/json");
//Send the request then wait here until it returns
yield return uwr.SendWebRequest();
if (uwr.isNetworkError)
{
Debug.Log("Error While Sending: " + uwr.error);
}
else
{
Debug.Log("Received: " + uwr.downloadHandler.text);
}
}
}