UIPrintInteractionController crashes when cancel

Question

I coded to print on the iPad and I've checked different documentation. My code seems to look right but when the controller is presented and I click on cancel it crashes with this error: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...

I tried adding a dispatch_async but didn't help. This is my code:

UIPrintInfo *printInfo = [UIPrintInfo printInfo];
printInfo.outputType = UIPrintInfoOutputGeneral;
printInfo.jobName = self.title;

    
UIPrintInteractionController *pc = [UIPrintInteractionController sharedPrintController];
pc.delegate = (id <UIPrintInteractionControllerDelegate>)self;
pc.printInfo = printInfo;

UIViewPrintFormatter *printFormatter = [self.webView viewPrintFormatter];
printFormatter.startPage = 0;
pc.printFormatter = printFormatter;



__weak SnaglistPrintViewController *weakSelf = self;

UIPrintInteractionCompletionHandler completionHandler =
^(UIPrintInteractionController *printController, BOOL completed,
  NSError *error) {
    dispatch_async( dispatch_get_main_queue (), ^{
    NSLog(@"Handler completed: %d", completed);
            if (!completed && error) {
              NSLog(@"FAILED! due to error in domain %@ with error code %u", error.domain, (unsigned int)error.code);
            }
            else {
                [weakSelf back];
            }
    } );
    };

if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {
    [pc presentFromRect:CGRectZero inView:self.view animated:YES completionHandler:completionHandler];


}