(Maybe not being paid is a factor?)
Such laws are generally not lawful in Europe. It certainly isn't here in Denmark, and I imagine it's the same in Sweden
I've had to insist on that clause being removed at a couple of places I've worked. As long as anything I build in my spare time (which I like to do a lot) isn't competitive.
That firm suggested that it was enforcable, and more so the closer your "invention" is to the business of your employee.
This, however, does not hold if your invention is patentable, since patentable inventions have special protection in Swedish law.
This is ironically not applicable to software, since we don't have software patents.