使用FBConnect iphone登录Facebook后，白色空白屏幕

- (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.