title: black screen appears after facebook authenticates , facebook dialog not shown
body: <p>i am integrating login through facebook in an ios app , in which during the app launch i show an alert to the user , and when the user clicks on the OK button on the alert view , then the FB login dialog is shown to the user .The problem is when i click on the home button and relaunch the app to show the alert, then after clicking on OK does not show the user the facebook login dialog. i have used the facebook ios sdk 3.0 and xcode 4.5 gm. *Interestingly when i kill the app from background every thing works fine.*below is the code in the app did finish launching for facebook </p>
  
  <pre><code>if (!self.session.isOpen)
  {
      NSArray *permissions = [[NSArray alloc] initWithObjects:
                              @"user_likes",
                              @"read_stream",@"publish_stream",@"email",
                              nil];
  
      self.session = [[FBSession alloc] initWithPermissions:permissions];
  
  
      if (self.session.state == FBSessionStateCreatedTokenLoaded)
  
      {
          // even though we had a cached token, we need to login to make the session usable
          [self.session openWithCompletionHandler:^(FBSession *session,
                                                    FBSessionState status,
                                                    NSError *error) {
      }];
      }
  }  
  </code></pre>
  
  <p>this is the method i call when user clicks on the OK button on the alert view</p>
  
  <pre><code>-(IBAction) facebookLogin
  {
  
  if (self.session.isOpen)
  {
      [self.session closeAndClearTokenInformation];        
  } 
  else
  {
      if (self.session.state != FBSessionStateCreated) {
          // Create a new, logged out session.
          self.session = [[FBSession alloc] init];
      }
  
  
      [self.session openWithCompletionHandler:^(FBSession *session,
                                                FBSessionState status,
                                                NSError *error) {
          if (error) {
              NSLog(@"dex is %@",error.description);
          }
      }];
  
      }
  }
  </code></pre>