Volley response 400 with null as message... what is null?

Question

I have been struggling with this problem for 3 days now and have received no help or answers that have worked from other posts on Stack.

I am trying to post variables from my mobile application to my server and everything is working fine on postman and I have double checked everything is mirroring from postman to my android http code.

I am using volley and am constantly receiving a status code 400 and have just recently got a message alongside the response code but this just says "Null"

What is null?? I see no errors in my code, it seems the headers are correct and same as postman and when debugged the JSON object is fine and getting the variables.

I am completely lost and would really appreciate some help:

My Code:

 private void sendRequest() {
    RequestQueue queue = Volley.newRequestQueue(this);
    String url ="http://xxx.xx.xx.xxx:8080/engAppApi/webservices/engineerTable/";

    final JSONObject jsonObject = new JSONObject();
    try {
        jsonObject.put("machineType", machineType);
        jsonObject.put("workOrderNumber", workOrderNumber);
        jsonObject.put("employeeName", employee);
        jsonObject.put("activity", activity);
        jsonObject.put("durationHours", durationHours);
        jsonObject.put("durationMins", durationMins);
        jsonObject.put("downtimeHours", downTimeHours);
        jsonObject.put("downtimeMins", downTimeMins);
        jsonObject.put("details", details);
        jsonObject.put("createdTimestamp", currentDateandTime);

    } catch (JSONException e) {
        // handle exception
    }
    JsonObjectRequest putRequest = new JsonObjectRequest(Request.Method.POST, url, jsonObject,
            new Response.Listener<JSONObject>()
            {
                @Override
                public void onResponse(JSONObject response) {
                    // response
                    Log.d("Response", response.toString());
                    Toast.makeText(MainActivity.this, response.toString(), Toast.LENGTH_SHORT).show();
                }
            },
            new Response.ErrorListener()
            {
                @Override
                public void onErrorResponse(VolleyError error) {
                    // error
                    Log.d("Error.Response", error.toString());
                    Log.d("errornetwork", String.valueOf(error.networkResponse));
                    Toast.makeText(MainActivity.this, "ERROR OCCURED", Toast.LENGTH_SHORT).show();
                }
            }
    ) {

        @Override
        public Map<String, String> getHeaders()
        {
            Map<String, String> headers = new HashMap<String, String>();
            headers.put("Content-Type", "application/json;");
            headers.put("securityToken", "Good Token");
            return headers;
            //  charset=utf-8
        }

        @Override
        public byte[] getBody() {
            try {
                Log.i("json", jsonObject.toString());
                return jsonObject.toString().getBytes("UTF-8");
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        public String getBodyContentType() {
            return "application/json";
        }
    };


    putRequest.setRetryPolicy(new DefaultRetryPolicy( 50000, 5, DefaultRetryPolicy.DEFAULT_BACKOFF_MULT));
    putRequest.setShouldCache(false);
    queue.add(putRequest);
}