I am following the Day 17, but there are some warning on the cosole when I run the app on simulator. Do I need to pay attention to those? They won't show up every time, only sometimes.

Snapshotting a view (0x10c051d80, _UIButtonBarStackView) that is not in a visible window requires afterScreenUpdates:YES.
Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
    (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
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