Exception thrown from UIPanel.FillAllDrawCalls

[Real World]

I'm getting the following exception on iOS occasionally in my menus. It's showing up in large numbers on Unitys performance reporting tool

MESSAGE

ArgumentException: texture is null
STACK TRACE

UIDrawCall.Create (System.String name, .UIPanel pan, UnityEngine.Material mat, UnityEngine.Texture tex, UnityEngine.Shader shader)
UIPanel.FillAllDrawCalls ()
UIPanel.UpdateSelf ()
UIPanel.LateUpdate ()

[ArenMook]

Why is the texture null? Trace which widget it's coming from that has no texture. That said, not sure why this would be causing an issue regardless. NGUI doesn't require a texture. Are you doing scene switching when this happens?

[Real World]

It's going to be difficult to trace from a call stack alone.
Whenever I have seen this personally it has been when tapping an input box. The error only happens once though and the game appears to render normally. We're not doing scene switching at this point (or at all anywhere in the app)