[TIMOB-2922] Android: Facebook Module NPE on Logout

Found in: #e1cb22a

1. Mashup > Facebook > Login/Logout
   
2. Login
   
3. Press back button
   
4. Login/Logout
   
5. Press Logout

I/ActivityManager(   85): Displayed activity com.appcelerator.titanium/org.appcelerator.titanium.TiActivity: 234 ms (total 234 ms)
D/KrollContext( 2004): (kroll$15) [1207,222604] Running evaluated script: app://examples/facebook_login_logout.js
D/dalvikvm( 2004): GC_FOR_MALLOC freed 25979 objects / 4384184 bytes in 71ms
D/LoginButtonProxy( 2004): (kroll$15) [99,222703] Second constructor called
D/Facebook-Util( 2004): GET URL: https://api.facebook.com/restserver.php?access_token=134793934930%7C0e5b712c915b0558ffc8b434-712472015%7ClUYHI9lZmgT2491I1vvVrnXabL8&;method=auth.expireSession&format=json
E/TiUIView( 2004): (main) [991,223694] TAP, TAP, TAP on [Ti.TiFacebookModuleLoginButton]
W/dalvikvm( 2004): threadid=28: thread exiting with uncaught exception (group=0x4001d7f0)
E/TiUncaughtHandler( 2004): (Thread-61) [280,223974] Sending event: exception on thread: Thread-61 msg:java.lang.NullPointerException; Titanium 1.6.0,2011/01/24 09:44,1cb22a
E/TiUncaughtHandler( 2004): java.lang.NullPointerException
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.LoginButton.updateButtonImage(LoginButton.java:94)
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.LoginButton$4.logout(LoginButton.java:136)
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.FacebookModule.fireLoginChange(FacebookModule.java:417)
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.FacebookModule.completeLogout(FacebookModule.java:313)
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.SessionListener.onLogoutFinish(SessionListener.java:37)
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.SessionEvents.onLogoutFinish(SessionEvents.java:115)
E/TiUncaughtHandler( 2004):   at ti.modules.titanium.facebook.FacebookModule$LogoutRequestListener.onComplete(FacebookModule.java:450)
E/TiUncaughtHandler( 2004):   at com.facebook.android.AsyncFacebookRunner$1.run(AsyncFacebookRunner.java:82)
D/TiApplication( 2004): (Thread-61) [13,223987] Analytics Event: type=