The object of type Sprite Renderer has been destroyed but you are still trying to access it

In general case it means that you have a

 SpriteRenderer renderer = GetComponent<Spriterenderer>();

or such, a variable referencing a SpriteRenderer.

Then you do a Destroy(renderer); or Application.LoadLevel() that causes unity to destroy the renderer and/or the gameobject, but you still have a reference to it (if you used LoadLevel, then the referencing variable is static or on an object that you called DontDestroyOnLoad() on).

After the renderer is destroyed, you still try to access it in some way like renderer.sprite = somesprite exactly lime the error says

This is basically a null reference error but Unity handles it differently for objects such as Components