Bug: Prediction: no channel selected before run would produce an error

[thao-do]

From Jianxu's student's feedback for Prediction - it seems the error message is not correct and there is no prompt for to correct the error:

If no channel is selected for an on-screen image, a "Dimensions not in output must be integers. Got NoneType for C." TypeError is thrown.

[derekthirstrup]

I found this error as well. Easiest workaround is to autodetect channel 0 if single channel input image or disable run button until the input image field is selected.