[iOS] <_UIAlertControllerActionSheetRegularPresentationController>) should have a non-nil sourceView or barButtonItem set before the presentation occurs

You aren’t initialising the activityViewController on iPad so it will always be nil.

Try:

- (void)shareLeaflet
{
    NSString *forwardedString = [[NSString alloc] initWithFormat:@"Check out this leaflet\n\n %@ \n\nself.theURLToShare];
    UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:[NSArray arrayWithObjects:forwardedString, nil] applicationActivities:nil];

    if (IDIOM == IPAD)
    {
        NSLog(@"iPad");
        activityViewController.popoverPresentationController.sourceView = self.view;
//        activityViewController.popoverPresentationController.sourceRect = self.frame;
        [self presentViewController:activityViewController
                           animated:YES
                         completion:nil];
    }
    else
    {
        NSLog(@"iPhone");
        [self presentViewController:activityViewController 
                          animated:YES 
                        completion:nil];
    }

And then to display it like in the image: (_shareBarButton is the UIBarButtonItem that you want the popover to display from)

- (void)shareLeaflet
    {
        NSString *forwardedString = [[NSString alloc] initWithFormat:@"Check out this leaflet\n\n %@ \n\nself.theURLToShare];
        UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:[NSArray arrayWithObjects:forwardedString, nil] applicationActivities:nil];

        if (IDIOM == IPAD)
        {
            NSLog(@"iPad");
            activityViewController.popoverPresentationController.sourceView = self.view;
    //        activityViewController.popoverPresentationController.sourceRect = self.frame;

           _popover = [[UIPopoverController alloc] initWithContentViewController:activityViewController];
           _popover.delegate = self;
           [_popover presentPopoverFromBarButtonItem:_shareBarButton permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
        }
        else
        {
            NSLog(@"iPhone");
            [self presentViewController:activityViewController 
                              animated:YES 
                            completion:nil];
        }

from:
https://stackoverflow.com/questions/33942902/uiactivityviewcontroller-crashing-on-ipad-with-sourceview-or-barbuttonitem