CrystalBall App crashes...

//(RCCrystalBall.h)
@interface RCCrystalBall : NSObject {
    NSArray *_predictions;
}

@property (strong, nonatomic, readonly) NSArray *predictions;

-(NSString*) randomPrediction;

@end

//(RCCrystalBall.m)

#import "RCCrystalBall.h"

@implementation RCCrystalBall

-(NSArray *) predictions {
    if (_predictions == nil){
        _predictions = [[NSArray alloc] initWithObjects:@"It is Certain", @"It is Decidedly so", @"All signs say YES", @"The stars are not aligned", @"The reply is no", @"It is doubtful", @"Better not tell you now", @"Concentrate and ask again", @"Unable to answer now", nil];
    }
    return _predictions;
}

-(NSString*) randomPrediction {
    int random = arc4random_uniform(self.predictions.count);
    return [self.predictions objectAtIndex:random];
}

@end

//(RCViewController.h)
#import <UIKit/UIKit.h>

@class RCCrystalBall;

@interface RCViewController : UIViewController
@property (strong, nonatomic) IBOutlet UILabel *predictionLabel;
@property (strong, nonatomic) RCCrystalBall *crystalBall;

-(void) makePrediction;

@end

//RCViewController.m

#import "RCViewController.h"
#import "RCCrystalBall.h"

@interface RCViewController ()

@end

@implementation RCViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.crystalBall = [[RCCrystalBall alloc] init];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Motion Events

-(void) makePrediction {
    self.predictionLabel.text = [self.crystalBall randomPrediction];
}

#pragma mark - Motion Events

-(void)motionBegan:(UIEventSubtype)motion withEvent:(UIEvent *)event {
    self.predictionLabel.text = nil;
}

-(void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event {
    if ( motion == UIEventSubtypeMotionShake ) {
        [self makePrediction];
    }
}

-(void)motionCancelled:(UIEventSubtype)motion withEvent:(UIEvent *)event {
    NSLog(@"motion cancelled");
}

#pragma mark - Touch Events

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    self.predictionLabel.text = nil;
}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    [self makePrediction];
}

-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"touches cancelled");
}

@end