Good that you were able to solve this issue.
You probably have the same problem as others with the mosaic.
- SNAP 11 error when mosaicking resampled Sentinel-2 L1C
- Mosaicking problem on SNAP 11 - Optical Toolbox - STEP Forum
This needs to be fixed by the developers.
A NullPointerException always indicates a developer mistake. Only in rare cases you can get around e.g., by selecting another option. So unfortunately, I think you need to wait till SNAP 12 or maybe the devs release an intermediate update. Which would be appropriate, I think, seeing that multiple have reported this issue already.