make swift style fix

HomeAssistant/Views/RootTabBarViewController.swift

@@ -211,7 +211,7 @@ class RootTabBarViewController: UITabBarController, UITabBarControllerDelegate {
                     let old = event.OldState! as Entity
                     var subtitleString = "\(new.FriendlyName!) is now \(new.State). It was \(old.State)"
                     if let uom = new.UnitOfMeasurement {
-                        subtitleString = "\(new.State) \(uom). It was \(old.State) \(old.UnitOfMeasurement)"
+                        subtitleString = "\(new.State) \(uom). It was \(old.State) \(String(describing: old.UnitOfMeasurement))"
                     }
                     let _ = Murmur(title: subtitleString)
                 }