Xcode 13 and iOS 15 began warning about missingAllowedClasses when using custom DataTransformers. There is very little documentation on custom DataTransformers, so I thought I would post a question and answer it here.

```
[general] *** -[NSKeyedUnarchiver _warnAboutPlistType:missingInAllowedClasses:] allowed unarchiving safe plist type ''NSString' (0x1dc9a7660) [/System/Library/Frameworks/Foundation.framework]', even though it was not explicitly included in the client allowed classes set: '{(
    "'NSArray' (0x1dc99c838) [/System/Library/Frameworks/CoreFoundation.framework]"
)}'. This will be disallowed in the future.

```

Notice in the warning message, it specifies 'NSArray' and the type missing, 'NSString'.