monitor.write(nil) doesn't error right. I had to add a bunch of .. "" to find where it was erroring.

I would say you're doing something wrong, perhaps consider creating an account on the ComputerCraft Forums and jumping onto the Ask a Pro section to get assistance.

monitor.write(nil) (because you forgot to initialize a variable somewhere) shows "Java threw an exception: java.lang.NullPointerException" (no line numbers or anything). Alternatively, monitor.write(nil .. "") causes an "attempt to concatenate nil and string" error, which includes line numbers, so you can actually see what's wrong instead of having to guess it.

This is already fixed in the latest beta