macOS - Trying to use a custom NSFormatter on a NSTextField fails miserably

I am trying to add a NSFormatter object to a NSTextField, so I can validate if the textfield carries on just alphanumeric strings.

So I do this:

1. I create a new Swift macOS app.
2. I add a NSTextField to the view controller
3. I add a custom Formatter to the view controller
4. I connect the text field's formatter outlet to the Formatter object, using interface builder.

I create this class and assign to the Formatter object.

FormatterTextNumbers.h

#import <Foundation/Foundation.h>
@import AppKit;


NS_ASSUME_NONNULL_BEGIN

@interface FormatterTextNumbers : NSFormatter

@end

NS_ASSUME_NONNULL_END

FormatterTextNumbers.m

#import "FormatterTextNumbers.h"

@implementation FormatterTextNumbers

- (BOOL)isAlphaNumeric:(NSString *)partialString
{
  static NSCharacterSet *nonAlphanumeric = nil;
  if (nonAlphanumeric == nil) {
  nonAlphanumeric = [NSCharacterSet characterSetWithCharactersInString:@"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890'. -"];
  nonAlphanumeric = [nonAlphanumeric invertedSet];
  }

  NSRange range = [partialString rangeOfCharacterFromSet:nonAlphanumeric];
    if (range.location != NSNotFound) {
      return NO;
    } else {
      return YES;
    }
}

- (BOOL)isPartialStringValid:(NSString *)partialString
            newEditingString:(NSString * _Nullable __autoreleasing *)newString
            errorDescription:(NSString * _Nullable __autoreleasing *)error {

  if ([partialString length] == 0) {
      return YES; // The empty string is okay (the user might just be deleting everything and starting over)
  } else if ([self isAlphaNumeric:partialString]) {
    *newString = partialString;
    return YES;
  }
  NSBeep();
  return NO;
}

You ask, why do I have these classes in Objective-C if my project uses Swift? Simple: if I create a subclass of the Formatter class using Swift, Xcode will not let me assign that subclass to the Formatter object. I need to create an Objective-C subclass of NSFormatter instead.

Said that, when I run the project, the text field disappears and I get this message, whatever this means:

Failure1[3071:136161] Failed to set (contentViewController) user defined inspected property on (NSWindow): *** -stringForObjectValue: only defined for abstract class. Define -[FormatterTextNumbers stringForObjectValue:]!

I remove the connection between the text field and the Formatter object and the app runs fine.

You have to define that method

From Apple documentation for NSFormatter (which is actually semi-abstract)

Summary

The default implementation of this method raises an exception.
Declaration

- (NSString *)stringForObjectValue:(id)obj;

Actually the same is for

- (BOOL)getObjectValue:(out id _Nullable * _Nullable)obj forString:(NSString *)string errorDescription:(out NSString * _Nullable * _Nullable)error;