Category on UIAlertView not being called

Question

I'm trying to use a category on UIAlertView to dismiss alert views when my app enters background state. I am implementing method swizzling (http://nshipster.com/method-swizzling/) to try to achieve this. The category methods are not being called on my UIAlertViews however.

Here is my category:

#import "UIAlertView+dismissalert.h"

#import <objc/runtime.h>

@implementation UIAlertView (dismissalert)

static NSHashTable *alertHashTable;

+ (void)load 
{

    SEL originalSelector = @selector(initWithContentViewController:);
    SEL replacementSelector = @selector(ts_initWithContentViewController:);

    Method originalMethod = class_getInstanceMethod([UIAlertView class], originalSelector);
    Method replacementMethod = class_getInstanceMethod([UIAlertView class], replacementSelector);

    method_exchangeImplementations(originalMethod, replacementMethod);

    [[NSNotificationCenter defaultCenter] addObserver: self
                                         selector: @selector(applicationDidEnterBackgroundNotification:)
                                             name: UIApplicationDidEnterBackgroundNotification
                                           object: nil];
}

- (id)ts_initWithContentViewController:(UIViewController *) contentViewController
{
    UIAlertView *alert = [self ts_initWithContentViewController: contentViewController];

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        alertHashTable = [NSHashTable weakObjectsHashTable];
    });

    [alertHashTable addObject:alert];

    return alert;
}

+ (void)applicationDidEnterBackgroundNotification: (NSNotification *) n 
{
    for (UIAlertView *alert in alertHashTable ) {
        if (alert.visible) {
            [alert dismissWithClickedButtonIndex:0 animated:NO];
        }
    }
}

I have implemented the UIAlertViewDelegate on my view controller.

Can anyone see why the category is not being called?

Thanks