One of my favorites in the UMP45 with red dot, and possibly a second attachment. However, when the other team EMPs the red dot is GONE and you have to estimate the center when firing. The best defense I can think of is to switch to your secondary. Even though Infinity Ward nerfed the akimbo FMG9, they are still pretty useful – and still over-used, in fact. At any rte, the point isn’t to set up akimbo FMG9s, but rather, if the loss of your red dot sight makes your shooting less effective, switch to your secondary.
So there’s the best defense against EMP loss of your red dot sight, change to your secondary weapon!