Crash on target lost / found continously in Vuforia sdk 2.0.31 and Unity 4.1.2

Vuforia SDK Version: Vuforia unity sdk 2.0.31 - Description with steps to reproduce:

Hello,

Please download the project source files from the below dropbox link. It contains the project source code files, and two text files in assets folder which explains the issue, steps to reproduce and untiy's editor log of the crash.

https://www.dropbox.com/s/a2j1cik4g8p1c24/ARGame_180413.zip 

Below is the text copied from the "Read me" file of the project in case if you cannot download that.

====================================================================Rigidbody null reference issue / crash, when target is lost / found continously repeatedly.in Vuforia 2.0.31 and Unity 4.1.2Ashish (18.April.2013)====================================================================        >>>    ISSUE:  --------------------------------------------------------    This project, if you run, will let you move a ball on a plane, according to you moving your target image.    But in Unity 4.1.2 it is crashing when the target is lost and visible repeateady quickly quickly.    Whereas In Unity 3.5.6 is working fine.        ("Repeatedly quickly quickly" means as fast as you can move in and out the image target from the camera view.        But keep it at that pace that it should alteast be be detectable.        In just 5-6 times of doing this, will not show you the real error or issue)                   >>>    ISSUE (In details), ITS CAUSE and TEMPORARY SOLUION :---------------------------------------------------------        1) While running the game, When the marker (trackerable image) continuesly appears and dis-appears    in front of the camera then the game crashes in iphone, giving me some Physics (PhyX) error in XCode console.        Below is the error give by Xcode console:                0xc472f4:  beq    0xc47310;                ContactMessage::onContactNotify(NxContactPair&, unsigned int) + 104 at PhysicsManager.cpp:230        2) When I do this same thing in Unity 4.1.2 PRO editor itself, on my mac. Then, It quites the unity program    itself. And when I checked the Editor log. It displayed 'Null Reference'        3) When I debuged my script Ball.cs by using couple of Debug.Logs, I found that it is giving me 'null refernce'    due to the current rigidbody object i.e my variable "_thisRigidbody"    (i.e ball's own rigidbody, that I assign in the awake function)        4) And If I comment all the two lines inside the PauseBallBehaviour() and ResumeBallBehaviour() functions.    Then this error doesn't happens. Which makes me even more sure that my _thisRigidbody variable is giving null reference.        5) SURPRIZINGLY. Its happens in unity 4.1.2 editor and xcode project build from Unity 4.1.2    either its pro or basic.    BUT. This issue does not happens in unity 3.5.6 editor and not even in xcode project build from Unity 3.5.6.        so, does that means UNTIY 4.1.2 is having some issue ?        or Vuforia SDK 2.0.31 is having some problem with Unity 4.1.2 ??        or Does I am doing something wrong with rigidbody or something else ???            >>>    Note:--------------------------------------------------------    a) If Xcode project build, is giving you any error regarding QCAR, then manually add the "QCARUnityPlayer.h"    to your xcode project, from Assets > Plugins > ios > QCARUnityPlayer.h        b) I am using  Unity 4.1.2 + ios Basic license, Xcode 4.6.1, OSX Lion  10.7.5, ipod Touch ios 5.1.1, iphone 3gs ios 6.1.2                            I am bady before this issue to solve it. But not lucky enough to see some success.    PLEASE REPLY ME IF YOU FIND ANY GOOD SOLUTION TO IT.  OR ANY SUGGESTION.        Thank you.     

- Development OS (Mac OS X, Windows, Linux): MAC OS X 10.7.5 - Mobile OS and Version: ios 5.1 and ios 6.1 - Mobile Device Manufacturer and Model name: ipod touch and iphone 3gs - Do the Vuforia Sample Applications show the same behavior?: I cannot test this issue in any vuforia unity samples, as no sample is there which deals with rigidbodies.