blockCrash.m

// https://stackoverflow.com/questions/23806751/strong-reference-to-a-weak-references-inside-blocks
__weak typeof(self) weakSelf = self;
void (^someBlock)(id) = ^(id data){
    if (weakSelf != nil) {
       // last remaining strong reference released by another thread. 
       // weakSelf is now set to nil.
       [myArray addObject:weakSelf];
    }
});