Core data random crash - iphone

Question

I finished the development of my app but at this point I face random crashes when I try to save an image in core data.
Here is the console message:
CoreData: error: Serious application error. Exception was caught
during Core Data change processing. This is usually a bug within an
observer of NSManagedObjectContextObjectsDidChangeNotification.
-[_PFExternalReferenceData compare:]: unrecognized selector sent to instance 0x94b69b0 with userInfo (null)
The problem is that it is non deterministic. It happens more or less once out of 5 times.
Would anyone have an idea where I should look for ?
EDIT:
Here is the code of my class that seems to pose a problem. It happens sometimes when I click on the save button, calling -(IBAction)saveResultingImage:
#import "ResultViewController.h"
#interface ResultViewController ()
#property (nonatomic)BOOL resultSaved;
#end
#implementation ResultViewController
#pragma mark - IBActions
- (IBAction)saveResultingImage
{
UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, NO, 0.0);
[self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
_resultImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
_resultSaved = [self saveOnCoreData];
[self performSegueWithIdentifier:#"BackToStartSegue" sender:self];
}
- (IBAction)discardWithoutSaving
{
[self performSegueWithIdentifier:#"BackToStartSegue" sender:self];
}
- (IBAction)restartDrawing:(id)sender
{
UIImage *backgroundImage = _subImage;
UIImageView *imageView =
[[UIImageView alloc] initWithFrame:CGRectMake(0, 0, backgroundImage.size.width, backgroundImage.size.height)];
imageView.image = backgroundImage;
[self.view addSubview:imageView];
UIImage *maskImage = _maskImage;
CGRect maskViewRect = CGRectMake(0, 0, maskImage.size.width, maskImage.size.height);
ImageMaskView *subView = [[ImageMaskView alloc] initWithFrame:maskViewRect image:maskImage];
[self.view addSubview:subView];
}
- (IBAction)showExplainVC
{
[self performSegueWithIdentifier:#"ShowInfoSegue" sender:self];
}
#pragma mark - Methods
- (BOOL)saveOnCoreData
{
NSManagedObjectContext *context = [self managedObjectContext];
NSManagedObject *newMeme =
[NSEntityDescription insertNewObjectForEntityForName:#"Meme" inManagedObjectContext:context];
NSData *toSaveMeme = UIImagePNGRepresentation(_resultImage);
[newMeme setValue:toSaveMeme forKey:#"image"];
NSError *error = nil;
if (![context save:&error]) {
DebugLog(#"Can't save: %# %#", error, [error localizedDescription]);
return NO;
}
return YES;
}
#pragma mark - View Controller LifeCycle
- (BOOL)shouldPerformSegueWithIdentifier:(NSString *)identifier sender:(id)sender
{
if ([identifier isEqualToString:#"BackToStartSegue"]) {
return _resultSaved;
}
return _resultSaved;
}
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
[self.navigationController setNavigationBarHidden:NO];
DebugLog(#"Called viewWillAppear");
self.navigationController.navigationBar.barStyle = UIBarStyleBlack;
}
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Custom initialization
}
return self;
}
- (void)viewDidLoad
{
[super viewDidLoad];
UIBarButtonItem *discardBarButtonItem =
[[UIBarButtonItem alloc] initWithTitle:#"Discard" style:UIBarButtonItemStylePlain target:self action:#selector(discardWithoutSaving)];
UIBarButtonItem *saveBarButtonItem =
[[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemSave target:self action:#selector(saveResultingImage)];
UIBarButtonItem *infoBarButton =
[[UIBarButtonItem alloc] initWithTitle:#"How to" style:UIBarButtonItemStylePlain target:self action:#selector(showExplainVC)];
discardBarButtonItem.style = UIBarButtonItemStyleBordered;
self.navigationItem.rightBarButtonItems = #[saveBarButtonItem, discardBarButtonItem, infoBarButton];
self.view.backgroundColor = [UIColor blackColor];
[self restartDrawing:nil];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#pragma mark - Core Data Stack
- (NSManagedObjectContext *)managedObjectContext
{
NSManagedObjectContext *context = nil;
id delegate = [[UIApplication sharedApplication] delegate];
if ([delegate performSelector:#selector(managedObjectContext)]) {
context = [delegate managedObjectContext];
}
return context;
}
#end
Thanks for showing interest :-)