If you have not unlocked the MPX from Ubisoft Club do not get it. Not sure if it is a predetermined weapon or is random but I unlocked it when I was level 10 and it is a blue level 3. I cannot dismantle it. It has to rot in my stash. I don't know if this is a bug or intended but I wouldn't bother grabbing it. I hope I'm able to get rid of it soon. Minor complaint I know but I felt like getting it out there.
We are aware of this issue, but thanks for taking the time to report it. A temporary workaround if you want to open the stash space is that it's possible to donate the weapon to a project.