Error while using Realm Object as a singleton. Is it a good idea?

I'm currently trying to use Realm in my app and I thought of using a subclass of RLMObject as a singleton to store the app state.

My first thought is if it's a very bad idea.

My AppState class has 2 main methods:

+ (instancetype)sharedInstance
{
    static dispatch_once_t once;
    static id sharedInstance = nil;

    RLMResults *result = [[self class] allObjects];

    if (result.count == 0) {
        dispatch_once(&once, ^{
            RLMRealm *realm = [RLMRealm defaultRealm];
            sharedInstance = [[self alloc] init];
            [realm beginWriteTransaction];
            [realm addObject:sharedInstance];
            [realm commitWriteTransaction];
        });
    }
    else {
        sharedInstance = [[[self class] allObjects] lastObject];
    }

    return sharedInstance;
}

- (void)update {
    RLMRealm *realm = self.realm;

    [realm beginWriteTransaction];
    [realm addOrUpdateObject:[[self class] sharedInstance]];
    [realm commitWriteTransaction];
}

I have properties for storing several params about the app state.

In one of my view controllers I get the following error:

'Attempting to modify object outside of a write transaction - call beginWriteTransaction on an RLMRealm instance first.'

The code snippet where the error occurs is as below:

AppState *defaultState = [AppState sharedInstance];
defaultState.appStateX = newAppStateValue; // This is where the app crashes.
[defaultState update];

I changed self.realm to [RLMRealm defaultRealm] in my -(void)update method. No dice!

I feel there is something fundamentally wrong with my understanding of Singletons and/or RLMObjects. Any help is appreciated.

---

Update

Based on Sakamoto's answer I modified my -(void)update method to accept a block as an argument. There is lot of room for error with this approach but I can go ahead and continue with my current implementation without making any drastic changes.

- (void)update:(void(^)(void))block {
    RLMRealm *realm = self.realm;

    [realm beginWriteTransaction];
    block();
    [realm commitWriteTransaction];
}

I update all the properties within the block.