Unassigned Reference Exception. How to create and add a texture 2D in the inspector?

I am trying to create a timer to the game. When i execute this code, it shows an Unassigned Reference Exception. I tried creating an image file and adding it to the timerbar in the inspector field, but was not able to do it. Please help me in creating an texture 2d (a monocolour rectangle would be enuf) file and adding it to the inspector.

void Start() { runTimer = true; }

  void Update() {
      if(runTimer  == true) {
             timer_cur -= Time.deltaTime;
             if(timer_cur  >= 0) {
                   timer_cur = timer_max
             }
      }
  }
 
  void OnGUI() {
        if(runTimer == true) {
             int pos_x = Screen.width / 2 - timerBar.width / 2;
             int pos_y = 100;
             int width = Mathf.RoundToInt(timer_cur / timer_max * timerBar.width);
             GUI.DrawTexture(new Rect(pos_x, pos_y, width , timerBar.height), timerBar);
        }
  }