EXC_BAD_ACCESS KERN_INVALID_ADDRESS on controllers dealloc after block returns

I've got a UIViewController with UICollectionView, implementing its delegate and dataSource. In 95% everything is working ok, but sometimes I get a crash (log, code etc below). I think it happens when successBlock of myOperation is executed in same moment as the controller gets deallocated (but I'm not sure).

Considering other question from SOF I assume myCollectionView == nil, however the delegate is still pointing somewhere (shouldn't it be changed, it's just assign property as far as I see). I have no idea how to fix such kind of issue, does anyone have any idea?

Crash log:

MyCollectionViewController.m line 0
__destroy_helper_block_
Crashed: com.apple.main-thread
EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x0000000a

Call stack:

libobjc.A.dylib 
objc_msgSend + 5
retain

UIKit   
-[UICollectionViewController collectionView] + 54

MyAppName   
MyCollectionViewController.m line 30
-[MyCollectionViewController dealloc]

MyAppName   
MyCollectionViewController.m line 0
__destroy_helper_block_

libsystem_blocks.dylib  
_Block_release + 216

MyAppName   
MyDownloaderController.m line 0
__destroy_helper_block_75

libsystem_blocks.dylib  
_Block_release + 216

MyAppName   
MyOperation.m line 0
__destroy_helper_block_19

libsystem_blocks.dylib  
_Block_release + 216

MyAppName   
MyOperation.m line 0
__destroy_helper_block_112

libsystem_blocks.dylib  
_Block_release + 216

UIKit   
UIApplicationMain + 1136

MyAppName
main.m line 15
main

Where lines around 30 in MyCollectionViewController:

- (void)dealloc {
    if ([self isViewLoaded] && self.collectionView) { // line 30
        self.collectionView.delegate = nil;
        self.collectionView.dataSource = nil;
        self.collectionView = nil;
    }
}

My successBlock:

MySuccessHandler successBlock = ^(UIImage *image) {
    if (weakCell) {
        [weakCell.pageImageView setImage:image];
    }
};