I've just found a bug in the new Xcode 8.0. So, I created the Vertical Stack View and added into it a few widgets. Then I tried to change margin for one of the widgets (Text field). I open the size Inspector and select "explicit" in menu Layout Margins and when I try to change any of field then my Xcode stops
Explicit layout Margins in Xcode 8.0
I have the same issue (Xcode 8 version 8.2.1 (8C1002)). It appears that any UI control other than Stack views will crash Xcode if you try and set explicit layout margins in Interface builder
yeah this ******* *****!
You should file a bug.