I tried to calculate the average of two bands in an image stack and got a java.lang.NullPointerException error message. It took me some time and the help of a colleague to understand that the image data was missing (no .img and .hdr files in the Stack.data folder). This is not really a bug but just to avoid confusion maybe it would be better to give a ‘data not found’ warning in these cases.
Best regards and thank you!