UITextView font is nil
Try checking the "selectable" checkbox for this UITextView in Interface Builder. It's in the Attributes Inspector. Per @VahramDodoryan's comment below, you can then set selectable to false if you don't want to support selection.
I can't explain why this works, but it's probably a UIKit bug. I had an IBOutlet to a UITextView whose font property was nil, and it would not respond to any font or text-color changes in code until after its text property had been set. I arrived at this solution through trial-and-error.
If you're still encountering this issue on recent releases of iOS, consider opening a radar:
I tried changing every property and constraint , I had on my text view.
Including selectable as suggested above
In frustration in the end I deleted and re-added it and that was the unsatisfying fix.
You should try logging self.detailDescriptionLabel. The value might be null.