title: unable to get the access token through [FBSDKAccessToken currentAccessToken] in iphone sdk
body: <p>I am creating a facebook app. I have implemented the login using class FBSDKLoginManager. At first time when i login the facebook, success block return the <strong>FBSDKLoginManagerLoginResult</strong> object which contain the <strong>access token object (FBSDKAccessToken object)</strong> which is use full for share the contain .But at second time (after relaunching the app) when I try to get the access token using the function  <strong>[FBSDKAccessToken currentAccessToken]</strong> return nil. So how to get the <strong>access token (FBSDKAccessToken object)</strong> second time without login facebook again.</p>