Extend ObjC class with Swift extension

Hi everybody. I have an application written in Objective-C. Until recently, I continued to maintain it and write new features in Objective-C. It was decided to switch to Swift and implement all new features on Swift. As a result, I made a bridge between Objective-C and Swift. Added new classes in swift and imported them via bridge header. One of the tasks required adding functionality to an existing Objective-C class, but I decided to implement this by extending the Objective-C class in a swift file.

Example Objective-C class

Settings.h

#import <Foundation/Foundation.h>

@interface Settings : NSObject

- (instancetype)initWithDictionary:(NSDictionary *)dictionary;

- (BOOL)boolForKey:(NSString *)key;

@end

Settings.m

#import "Settings.h"

@implementation Settings {
    NSDictionary *_dictionary;
}

- (instancetype)initWithDictionary:(NSDictionary *)dictionary {
    self = [super init];
    if (self) {
        _dictionary = dictionary;
    }
    return self;
}

- (BOOL)boolForKey:(NSString *)key {
    return [[_dictionary objectForKey:key] boolValue];
}
@end

Swift extension

Settings+extension.swift

import Foundation
extension Settings {

    @objc public func isFirstProperty() -> Bool {
        return self.bool(forKey: "isFirstProperty")
    }

    @objc public func isSecondProperty() -> Bool {
        return self.bool(forKey: "isSecondProperty")
    }

}

Call example inside UICollectionViewController sub class

@implementation CollectionViewController {
    Settings* _settings;
}
-(void)viewDidLoad {
    [super viewDidLoad];
    _settings = [[Settings alloc] initWithDictionary:someDictionary];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"reuseIdentifier" forIndexPath:indexPath];
if ([_settings isFirstProperty]) {
//Do something with cell according to firstProperty
} else if ([_settings isSecondProperty]) {
//Do something with cell according to secondProperty
} 

return cell;
}

@end

I tested the app, everything worked great. I uploaded the app to the App Store. Almost from the beginning of the use of the new application by users, I began to receive crash reports.

Crash report

I could not understand what was the matter. But after I removed the Swift extensions and implemented them directly in the Objective-C class, the crashes stopped.

After removing Settings+extension.swift and implementing inside Settings.m/h

Example Objective-C class

Settings.h

#import <Foundation/Foundation.h>

@interface Settings : NSObject

- (instancetype)initWithDictionary:(NSDictionary *)dictionary;

- (BOOL)boolForKey:(NSString *)key;

- (BOOL)isFirstProperty;

- (BOOL)isSecondProperty;
@end

Settings.m

#import "Settings.h"

@implementation Settings {
    NSDictionary *_dictionary;
}

- (instancetype)initWithDictionary:(NSDictionary *)dictionary {
    self = [super init];
    if (self) {
        _dictionary = dictionary;
    }
    return self;
}

- (BOOL)isFirstProperty {
    return [[_dictionary objectForKey:key] boolValue];
}

- (BOOL)isSecondProperty {
    return [[_dictionary objectForKey:key] boolValue];
}


@end

1. Can somebody explain, why I had crashes?

2. Is it ok to extend Objective-C classes like that?

3. What kind of issues I can expect during migration to swift ?from Objective-C? (Any literature that explore this topic?)

Thanks in advance!