i  made simple cloud recognition App from follow post.

https://library.vuforia.com/articles/Solution/How-To-Create-a-Simple-Cloud-Recognition-App-in-Unity

but i'm having problem.i register 2 targets on my cloud detabase.

1.play on Unity

2.put on 1st target on front of camera.program find 1st target.3D Cube object is shown.no problem at this moment

3.1st target is out of camera.

4.put on 2nd target. following error occurred.

Exception in callback: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> UnityEngine.MissingReferenceException: The object of type 'ImageTargetBehaviour' has been destroyed but you are still trying to access it.

Your script should either check if it is null or you should not destroy the object.

  at (wrapper managed-to-native) UnityEngine.Behaviour:get_enabled ()

  at Vuforia.StateManagerImpl.UpdateTrackablePoses (UnityEngine.Transform arCameraTransform, .TrackableResultData[] trackableResultDataArray, .VuMarkTargetResultData[] vuMarkResultDataArray, TrackableIdPair originTrackableID, Int32 frameIndex) [0x00000] in <filename unknown>:0

  at Vuforia.VuforiaManagerImpl.UpdateTrackers (FrameState frameState) [0x00000] in <filename unknown>:0

  at Vuforia.VuforiaManagerImpl.Update (ScreenOrientation counterRotation, Boolean reapplyOldState) [0x00000] in <filename unknown>:0

  at Vuforia.VuforiaARController.UpdateStatePrivate (Boolean forceUpdate, Boolean reapplyOldState) [0x00000] in <filename unknown>:0

  at Vuforia.VuforiaARController.UpdateState (Boolean forceUpdate, Boolean reapplyOldState) [0x00000] in <filename unknown>:0

  at Vuforia.DigitalEyewearARController.Update () [0x00000] in <filename unknown>:0

  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (object,object[],System.Exception&)

  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000d0] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222

  --- End of inner exception stack trace ---

  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000eb] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232

  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115

  at System.Delegate.DynamicInvokeImpl (System.Object[] args) [0x000b9] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Delegate.cs:443

  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x00018] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/MulticastDelegate.cs:71

  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System/Delegate.cs:415

  at Vuforia.DelegateHelper.InvokeDelegate (System.Delegate action, System.Object[] args) [0x00000] in <filename unknown>:0

UnityEngine.Debug:LogError(Object)

Vuforia.DelegateHelper:InvokeDelegate(Delegate, Object[])

Vuforia.DelegateHelper:InvokeWithExceptionHandling(Action)

Vuforia.VuforiaAbstractBehaviour:Update()

any help would be appreciated!