In programming, a null pointer is an error caused by using a structure called a pointer without pointing it anywhere first. See, when computer programs need information, you have to give it to them. Only, instead of giving it to them, you can also pass them a note that says "Look over here for the information you want." This is useful because it allows you to access things that you otherwise wouldn't, like a library book in the other town. The "look over there" thing is called a pointer.
However, if you don't tell it where "over here" is when you do this, it doesn't have the ability to question you. It just goes "Oh, okay," wanders off, searches around for the information, and sooner or later gets gunned down in the bad part of town. This kind of error is a segmentation fault.
It's probably bad to send it anywhere outside of your immediate control, because there's a lot of bad parts of town in a running computer system. Other programs tend to have paranoid police, and the operating system is especially out of its mind, because it's been reading Stalin's writings while drinking.
Anyway, I'm straining this metaphor awfully thin at this point, but when you see that damned "illegal operation" box, or the words "segmentation fault" on your computer, and you lost hours of work? This is what probably happened. Yes, it's annoying. Yes, we're trying to make it never happen again.