Firebase – get Application Context error 'Context.getApplicationContext()' on a null object reference

I am new to android development. I couldn’t find and so posting here.
I want to send firebase push notification to all user through using admin app when implementation is done i am getting error getApplicationContext() on a null object reference both class Push_Notification_All and class MySingleton if someone know the solution please add here

getApplicationContext()’ on a null object reference
In Activity Push_Notification_All and class MySingleton

 public class Push_Notification_All extends AppCompatActivity {

    private final String FCM_API = "https://fcm.googleapis.com/fcm/send";
    String NOTIFICATION_MESSAGE;
    String NOTIFICATION_TITLE;
    final String TAG = "NOTIFICATION TAG";
    String TOPIC;
    private final String contentType = "application/json";
    final Context context = this;
    EditText edtMessage;
    EditText edtTitle;
    Dialog loading;
    ProgressBar progressBar;
    private final String serverKey = "key=ABBBAdfgfgzyw:APA91bFfdgdfgAFGveSeXnCNpglbKvpbd9-zu538Z1kA-QdMA41gRb2bgSXcu-2jlk1vhnY8ajXor-PYglN5H68ECTNqHeabfACfD8amSFRIghvGXsmIgEi6Ykw6Jlv2ihhdghfhhgBPr";


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_push__notification__all);

        edtTitle = (EditText) findViewById(R.id.edtTitle);
        edtMessage = (EditText) findViewById(R.id.edtMessage);
        loading = new Dialog(context);
        loading.setContentView(R.layout.loading);
        progressBar = (ProgressBar) loading.findViewById(R.id.spn_1);
        ((Button) findViewById(R.id.btnSend)).setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                if (!AppStatus.getInstance(Push_Notification_All.this).isOnline()) {
                    Toast.makeText(Push_Notification_All.this, "Internet is not Available", Toast.LENGTH_SHORT).show();
                    return;
                }
                Push_Notification_All.this.loading.show();
                Push_Notification_All.this.loading.setCancelable(false);
                Push_Notification_All.this.progressBar.setIndeterminateDrawable(new Circle());
                Push_Notification_All push_Notification_All = Push_Notification_All.this;
                push_Notification_All.TOPIC = "/topics/userABC";
                push_Notification_All.NOTIFICATION_TITLE = push_Notification_All.edtTitle.getText().toString();
                Push_Notification_All push_Notification_All2 = Push_Notification_All.this;
                push_Notification_All2.NOTIFICATION_MESSAGE = push_Notification_All2.edtMessage.getText().toString();
                JSONObject jSONObject = new JSONObject();
                JSONObject jSONObject2 = new JSONObject();
                try {
                    jSONObject2.put("title", Push_Notification_All.this.NOTIFICATION_TITLE);
                    jSONObject2.put("message", Push_Notification_All.this.NOTIFICATION_MESSAGE);
                    jSONObject.put("to", Push_Notification_All.this.TOPIC);
                    jSONObject.put("data", jSONObject2);
                } catch (JSONException e) {
                    Push_Notification_All.this.loading.dismiss();
                    Log.e(TAG, "onCreate: " + e.getMessage());
                }
                Push_Notification_All.this.sendNotification(jSONObject);
            }
        });
    }
    public void sendNotification(JSONObject jSONObject) {
        MySingleton.getInstance(Push_Notification_All.this).addToRequestQueue(new JsonObjectRequest(FCM_API, jSONObject, new Response.Listener<JSONObject>() {
            public void onResponse(JSONObject jSONObject) {
                Log.i(TAG, "onResponse: " + jSONObject.toString());
                Push_Notification_All.this.edtTitle.setText("");
                Push_Notification_All.this.edtMessage.setText("");
                Push_Notification_All.this.loading.dismiss();
            }
        }, new Response.ErrorListener() {
            public void onErrorResponse(VolleyError volleyError) {
                Push_Notification_All.this.loading.dismiss();
                Toast.makeText(Push_Notification_All.this, "Request error", Toast.LENGTH_LONG).show();
                Log.i(TAG, "onErrorResponse: Didn't work");
            }
        }) {
            public Map<String, String> getHeaders() throws AuthFailureError {
                HashMap hashMap = new HashMap();
                hashMap.put("Authorization", serverKey);
                hashMap.put("Content-Type", contentType);
                return hashMap;
            }
        });
    }

}

MySingleton

  public class MySingleton {
    private static MySingleton instance;
    private Context ctx;
    private RequestQueue requestQueue = getRequestQueue();

    private MySingleton(Context context) {
        ctx = context;
    }

    public static synchronized MySingleton getInstance(Context context) {
        MySingleton mySingleton;
        synchronized (MySingleton.class) {
            if (instance == null) {
                instance = new MySingleton(context);
            }
            mySingleton = instance;
        }
        return mySingleton;
    }

    public RequestQueue getRequestQueue() {
        if (requestQueue == null) {
            requestQueue = Volley.newRequestQueue(ctx.getApplicationContext());
        }
        return requestQueue;
    }

    public <T> void addToRequestQueue(Request<T> request) {
        getRequestQueue().add(request);
    }
}