Having issues getting this runing with the the JSON 6 library.

[vangalvin]

A quick look in to it and it appears that there is a compatibility issue with the new
version 6 JSON library.

[pippin88]

spiffs_rest_api_nonblocking

Change:

    //build json object of program data
    StaticJsonBuffer<200> jsonBuffer;
    JsonObject &json = jsonBuffer.createObject();
    json["x"] = x;
    json["y"] = y;

    char jsonchar[200];
    json.printTo(jsonchar); //print to char array, takes more memory but sends in one piece
    server.send(200, "application/json", jsonchar);

To:

    //build json object of program data
    StaticJsonDocument<200> jsonBuffer;
    JsonObject json = jsonBuffer.to<JsonObject>();
    json["x"] = x;
    json["y"] = y;

    char jsonchar[200];
    serializeJson(jsonBuffer, jsonchar); //print to char array, takes more memory but sends in one piece
    server.send(200, "application/json", jsonchar);