After whole day testing, I found the Fatal Error only happened in Unity Play mode, but it can be run in iPhone.
And Thank you for "udvaritibor" 's script shown at below, since that script can be used in Unity when developing the effect in Unity editor ( at least can turn on the AR camera for testing/ preview, but don't try to change any scene in unity editor).
Once confirmed all layout I will activate the AR camera, uncheck the delay initialization, and move all image target stuff out of the game object with "udvaritibor" 's script before build to the mobile phone for testing.
this is what i found and hope Vuforia can fix this issue as soon as possible. and if anyone have any news or solution, please feel free to share.
*I found the vuforia 6 is not support 2 AR CAMERA in APP? Since I have used the previous version ( seem like ver.5-5-9) to develop an APP which can include 2 AR cameras, which mean 1 button with AR camera can control the front camera, and 1 button can control the back camera for the user selfie.
But the vuforia 6 seem only support 1 AR camera in APP. i have tried 2 scene with 2 AR cameras in unity , but when i set one AR Camera use front camera and other one use back camera, the result i got is both 2 AR camera have 2 same setting, it not able to be set individually..... do anyone also have same issues? or any solution? or can the Vuforia team can add back this feature?