It shows some error related to some library of SwiftFoundation. I was not sure about this error. So first thing what I did is report it to apple and then tried few things like. Restarting simulator couple of times and restarting Xcode couple of times. But it didn't work. So may be it's related to SDK update. I updated the latest SDK but still it was not working. So at last I give it to Apple to solve the problem but I needed that screen shot. So here is alternate way to take Screenshot of iPhone simulator.
With simulator running. Select Go to Edit menu and Select Copy Screen.
This will copy current screen of simulator. Now open the preview and go to File and Select New From Clipboard.
And it will give you new image with copied screen of your simulator, now save it and use it with Preview. Hope this helps you.