White blank screen after login in Facebook with FBConnect iphone

I am using FBConnect API...And I am using this code while pressing button..

- (IBAction)postGradesTapped:(id)sender {
_posting = YES;
// If we're not logged in, log in first...
if (![_session isConnected]) {
    self.loginDialog = nil;
    _loginDialog = [[FBLoginDialog alloc] init];    
    [_loginDialog show];    
}
// If we have a session and a name, post to the wall!
else if (_facebookName != nil) {
    [self postToWall];
}
// Otherwise, we don't have a name yet, just wait for that to come through.

}

Now when I press the button...

White blank screen Comes up.开发者_高级运维..:(

What can be the problem??

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As per your code, on login success you are posting on your wall. So it may be getting blank screen. Try print something on console on login success.