Without further investigation, it is difficult to know exactly what went wrong. It could be a kernel bug, or it could be faulty hardware (eg. bad RAM), overheating, or even a corrupt file system.
The NULL pointer deference simply means that kernel code was executing and an instruction tried to use memory location 0 as an offset to where additional instruction data is stored. This occurs under any of a number of different circumstances, but *should* never happen. Unfortunately, due to bugs or hardware issues, it does happen occasionally.
Standard advice, continue to watch for repeat problems - if it does repeat, then it is time to start narrowing down the possibilities, through research, analysys, trial and error, and often a bit of luck.