error CS0161: not all code paths return a value

I have a problem with small code: error CS0161: not all code paths return a value. I'm a beginner in coding and please help me.

 private IEnumerator InitializeBubblesAgain()
 {
     for (int column = 0; column < BubbleColumns; column++)
     {
         for (int row = 0; row < BubbleRows; row++)
         {
             MyMaterial material = MyMaterial.GetRandomMaterial(); //get a random color
             //create a new bubble
             var go = (GameObject)Instantiate(BubbleParameter,
                                              new Vector3((float)column * BubbleParameter.transform.localScale.x,
                         (float)row * BubbleParameter.transform.localScale.y, 0f), Quaternion.identity);
             go.tag = material.ColorName;
             BubblesArray[column, row] = new Bubble(go, material);
             go.name = column.ToString() + "-" + row.ToString();
             
             var renderer = go.transform.renderer;
             renderer.material = material; //set the color
             
             
         }
     }
 }