i think undestructive should be a word. because not everything that is not destructive is constructive.

using this definition:

con·struc·tive adj. 1. Serving to improve or advance; helpful.

i could have a habit or hobby that is not constructive (as in to improve or help) but is at the same time the opposite (as in not damaging). and this is not the same as nondestructive. undestructive sounds cooler.