I keep getting alertview in signupviewcontroller

//
//  SignUpViewControllerLeverancier.m
//  SIKA
//
//  Created by BLANCO on 25-04-14.
//  Copyright (c) 2014 Sinan Karakurt. All rights reserved.
//

#import "SignUpViewControllerLeverancier.h"
#import <Parse/Parse.h>

@interface SignUpViewControllerLeverancier ()

@end

@implementation SignUpViewControllerLeverancier


- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}



- (IBAction)signupLeverancier:(id)sender {

    NSString *naamLeverancier = self.naamLeverancierField.text ;

    NSString *telefoonLeverancier = [self.telefoonLeverancierField.text stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];

    NSString *bedrijfsnaamLeverancier = self.bedrijfsnaamLeverancierField.text;

    NSString *adresLeverancier = self.adresLeverancierField.text;

    NSString *postcodeLeverancier = self.postcodeLeverancierField.text;

    NSString *stadLeverancier = self.stadLeverancierField.text;

    NSString *emailLeverancier = [self.emailLeverancierField.text stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];

    NSString *wachtwoordLeverancier = [self.wachtwoordLeverancierField.text stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];

    if  ([naamLeverancier length] == 0 ||
         [telefoonLeverancier length] == 0 ||
         [bedrijfsnaamLeverancier length] == 0 ||
         [adresLeverancier length] == 0 ||
         [postcodeLeverancier length] == 0 ||
         [stadLeverancier length] == 0 ||
         [emailLeverancier length] == 0 ||
         [wachtwoordLeverancier length] == 0)
    {

        UIAlertView *alertView = [[ UIAlertView alloc] initWithTitle:@"Leeg veld" message:@"Vul de lege velden in" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil];

        [alertView show];
            }

 else {
PFUser *newUserLeverancier = [PFUser user];
newUserLeverancier[@"naamLeverancier"] = naamLeverancier;
        newUserLeverancier[@"telefoonLeverancier"]= telefoonLeverancier;
        newUserLeverancier.username = bedrijfsnaamLeverancier;
        newUserLeverancier[@"adresLeverancier"] = adresLeverancier;
        newUserLeverancier[@"postcodeLeverancier"] = postcodeLeverancier;
        newUserLeverancier[@"stadLeverancier"] = stadLeverancier;
        newUserLeverancier.email = emailLeverancier;
        newUserLeverancier.password = wachtwoordLeverancier;

       [newUserLeverancier signUpInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
            if (error) {
               UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Sorry!" message:[error.userInfo objectForKey:@"error"] delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
                [alertView show];
            }
           else {
                [self.navigationController popToRootViewControllerAnimated:YES];
            }
        }];
 }
}
@end