You need to specify nullable also for the handlers/blocks

- (void)setObject:(nullable id)value
forKey:(nonnull NSString *)defaultName
objectChanged:(nullable void(^)(NSUserDefaults *userDefaults, id value))changeHandler
objectRamains:(nullable void(^)(NSUserDefaults *userDefaults, id value))remainHandler;

Why? It's due to Swift. Swift allows optional parameters (?), which Objective-C does not. This is done as a bridge between the both of them for the Swift compiler to know those parameters are optional. A 'Nonnull' will tell the Swift compiler that the argument is the required. A nullable that it is optional

For more info read: https://developer.apple.com/swift/blog/?id=25

The correct, working method declaration, accepted by compiler:

- (void)setObject:(nullable id)value
forKey:(nonnull NSString *)defaultName
objectChanged:(nullable void(^)(NSUserDefaults *_Nonnull userDefaults, id _Nullable value))changeHandler
objectRamains:(nullable void(^)(NSUserDefaults *_Nonnull userDefaults, id _Nullable value))remainHandler;

You can use the following macros around blocks of declarations (functions and variables) in objective c headers:

NS_ASSUME_NONNULL_BEGIN


NS_ASSUME_NONNULL_END

You need to then add nullable annotations for references that can be nil within that block. This applies to both function parameters and variable declarations.

As in:

@interface SMLBaseUserDetailsVC : UIViewController < UICollectionViewDelegate>
NS_ASSUME_NONNULL_BEGIN


@property (nonatomic, readonly) IBOutlet UIScrollView *detailsScrollView;
@property (nonatomic, readonly) IBOutlet UICollectionView *photoCV;
@property (nonatomic, weak, readonly) SMLUser *user;
- (IBAction)flagUser:(id)sender;
- (IBAction)closeAction:(nullable id)sender;
- (void) prefetchPhotos;


NS_ASSUME_NONNULL_END


@end

Edit* why do we have to do this? For an objective-c class to be interoperable with swift, you need to declare nullability so that the compiler knows to treat properties as swift optionals or not. Nullable objective c properties are known as optionals in swift and using these macros in conjunction with the nullable declarators for properties allows the compiler to treat them as optionals (Optionals are objects that wrap up either the object or nil).

I post this answer to tell why should add _Nonnull or nullable.

According to this blog: https://developer.apple.com/swift/blog/?id=25

One of the great things about Swift is that it transparently interoperates with Objective-C code, both existing frameworks written in Objective-C and code in your app. However, in Swift there’s a strong distinction between optional and non-optional references, e.g. NSView vs. NSView?, while Objective-C represents boths of these two types as NSView *. Because the Swift compiler can’t be sure whether a particular NSView * is optional or not, the type is brought into Swift as an implicitly unwrapped optional, NSView!.

it's all for Swift.

To disable this warning across your entire project

1. Go to Build Settings

2. Make sure you are viewing the "All" tab not "Basic" or "Customized"

3. Search for the field Other Warning Flags.

4. Add the following flag: -Wno-nullability-completeness.

5. Now clean and build (⇧⌘K then ⌘R)

6. Quit Xcode and re-open it (Don't skip this step!)

   Note: It can take a few seconds for the warnings to go away even after you finish building and relaunching Xcode, for me it takes about 15 seconds for Xcode to fully update.

Remove below macro from your .h file and warning will disappear

NS_ASSUME_NONNULL_BEGIN


NS_ASSUME_NONNULL_END