Necroman strikes again! Just to say that if you use null as the value for a sprite field or for a FsmObject:sprite, you'll still get the error talked about earlier (wawa CastException wawa...).
So if you need to set sprites to something empty, just create such a sprite (2 x 2 pixels for example) with an alpha of 0 and add it to your sprite sheet, then use that "empty" sprite instead of null.
Also, don't forget that the uGUI image element will keep its former size when you set the image to this empty sprite, so depending on your needs you may need to set your image component to its native size.
Note that there's no no action to do that right now so you'll have to use "Call Method", UnityEngine.UI.Image with "void SetNativeSize ()".
P.S.: might it be possible to have an action on the ecosystem to set a uGUI sprite to its native size? Thx.