title: FBSDKProfile currentProfile is nil but FBSDKAccessToken currentAccessToken has value
body: <p>Just like what the title says, is this a bug or is it really possible? I use the <code>[FBSDKAccessToken currentAccessToken].tokenString</code> to check whether an existing Facebook session is existing, then afterwards I'll use the <code>[FBSDKProfile currentProfile].userID</code> to get the session's userId. 
  However sometimes I encounter that <code>[FBSDKAccessToken currentAccessToken].tokenString</code> has a value but <code>[FBSDKProfile currentProfile]</code> is nil.</p>
  
  <p>I use this for the auto-login feature of my application.</p>
  
  <p>Thanks for your response! </p>
  
  <p>So I have this code snippet:</p>
  
  <pre><code>    if ([FBSDKAccessToken currentAccessToken].tokenString){
  //proceed to auto login since Facebook is still logged in
          NSLog(@"Facebook user id: %@",[FBSDKProfile currentProfile].userID)
      }
  </code></pre>
  
  <p>The return of that log is nil.</p>