Null.Pointer Exception

I am currently trying to create a classification of polarimetric data. Whenever I want to run the supervised classification I receive the error message that the calculation can´t be run due to “null pointer exception”. Does anyone know what it means and how to solve it?

Thanks in advance!

what data are you using?
what were the steps to pre-process the data?
which classification did you select?
what kind of training areas are you using?

Meawhile I found a way of how to bypass the problem. I used Band Math to create a mask of the area.
Thank you for helping!