ESP32 wifi manager and custom parameter SPIFFS

wifimanager-esp32.ino

#include <FS.h>          // this needs to be first, or it all crashes and burns...
#include <WiFiManager.h> // https://github.com/tzapu/WiFiManager
#include <ArduinoJson.h> // https://github.com/bblanchon/ArduinoJson

#ifdef ESP32
  #include <SPIFFS.h>
#endif

//define your default values here, if there are different values in config.json, they are overwritten.
char mqtt_server[40];
char mqtt_port[6]  = "8080";
char api_token[32] = "YOUR_API_TOKEN";

//default custom static IP
char static_ip[16] = "10.0.1.56";
char static_gw[16] = "10.0.1.1";
char static_sn[16] = "255.255.255.0";

//flag for saving data
bool shouldSaveConfig = true;

//callback notifying us of the need to save config
void saveConfigCallback () {
  Serial.println("Should save config");
  shouldSaveConfig = true;
}

void setupSpiffs(){
  //clean FS, for testing
  //SPIFFS.format();

  //read configuration from FS json
  Serial.println("mounting FS...");

  if (SPIFFS.begin()) {
    Serial.println("mounted file system");
    if (SPIFFS.exists("/config.json")) {
      //file exists, reading and loading
      Serial.println("reading config file");
      File configFile = SPIFFS.open("/config.json");
      if (configFile) {
        Serial.println("opened config file");
        size_t size = configFile.size();
        // Allocate a buffer to store contents of the file.
        std::unique_ptr<char[]> buf(new char[size]);

        configFile.readBytes(buf.get(), size);
        DynamicJsonBuffer jsonBuffer;
        JsonObject& json = jsonBuffer.parseObject(buf.get());
        json.printTo(Serial);
        if (json.success()) {
          Serial.println("\nparsed json");

          strcpy(mqtt_server, json["mqtt_server"]);
          strcpy(mqtt_port, json["mqtt_port"]);
          strcpy(api_token, json["api_token"]);

          // if(json["ip"]) {
          //   Serial.println("setting custom ip from config");
          //   strcpy(static_ip, json["ip"]);
          //   strcpy(static_gw, json["gateway"]);
          //   strcpy(static_sn, json["subnet"]);
          //   Serial.println(static_ip);
          // } else {
          //   Serial.println("no custom ip in config");
          // }

        } else {
          Serial.println("failed to load json config");
        }
      }
    }
  } else {
    Serial.println("failed to mount FS");
  }
  //end read
}

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println();

  setupSpiffs();

  // WiFiManager, Local intialization. Once its business is done, there is no need to keep it around
  WiFiManager wm;

  //set config save notify callback
  wm.setSaveConfigCallback(saveConfigCallback);

  // setup custom parameters
  // 
  // The extra parameters to be configured (can be either global or just in the setup)
  // After connecting, parameter.getValue() will get you the configured value
  // id/name placeholder/prompt default length
  WiFiManagerParameter custom_mqtt_server("server", "mqtt server", mqtt_server, 40);
  WiFiManagerParameter custom_mqtt_port("port", "mqtt port", mqtt_port, 6);
  WiFiManagerParameter custom_api_token("api", "api token", api_token, 32);

  //add all your parameters here
  wm.addParameter(&custom_mqtt_server);
  wm.addParameter(&custom_mqtt_port);
  wm.addParameter(&custom_api_token);

  // set static ip
  // IPAddress _ip,_gw,_sn;
  // _ip.fromString(static_ip);
  // _gw.fromString(static_gw);
  // _sn.fromString(static_sn);
  // wm.setSTAStaticIPConfig(_ip, _gw, _sn);

  //reset settings - wipe credentials for testing
  //wm.resetSettings();

  //automatically connect using saved credentials if they exist
  //If connection fails it starts an access point with the specified name
  //here  "AutoConnectAP" if empty will auto generate basedcon chipid, if password is blank it will be anonymous
  //and goes into a blocking loop awaiting configuration
  if (!wm.autoConnect()) {
    Serial.println("failed to connect and hit timeout");
    delay(3000);
    // if we still have not connected restart and try all over again
    ESP.restart();
    delay(5000);
  }

  // always start configportal for a little while
  wm.setConfigPortalTimeout(60);
  wm.startConfigPortal();

  //if you get here you have connected to the WiFi
  Serial.println("connected...yeey :)");

  //read updated parameters
  strcpy(mqtt_server, custom_mqtt_server.getValue());
  strcpy(mqtt_port, custom_mqtt_port.getValue());
  strcpy(api_token, custom_api_token.getValue());

  //save the custom parameters to FS
  if (shouldSaveConfig) {
    Serial.println("saving config");
    DynamicJsonBuffer jsonBuffer;
    JsonObject& json = jsonBuffer.createObject();
    json["mqtt_server"] = mqtt_server;
    json["mqtt_port"]   = mqtt_port;
    json["api_token"]   = api_token;

    // json["ip"]          = WiFi.localIP().toString();
    // json["gateway"]     = WiFi.gatewayIP().toString();
    // json["subnet"]      = WiFi.subnetMask().toString();

    File configFile = SPIFFS.open("/config.json", FILE_WRITE);
    if (!configFile) {
      Serial.println("failed to open config file for writing");
    }

    json.prettyPrintTo(Serial);
    json.printTo(configFile);
    configFile.close();
    //end save
    shouldSaveConfig = false;
  }

  Serial.println("local ip");
  Serial.println(WiFi.localIP());
  Serial.println(WiFi.gatewayIP());
  Serial.println(WiFi.subnetMask());
}

void loop() {
  // put your main code here, to run repeatedly:


}

Not able to save the data. showing "failed to open config file for writing" error every time. Can anyone tell me how to save .json file in esp32. It works perfectly in esp8266 but failing in esp32.