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VUFORIA restart problem

hello every one

I made my appli with unity android and vuforia and everything works well.

but when i stop my appli , do somothing or not, and restart my appli, the ARcam of my appli doesnt film any image and so, dont work : dont see and dont recognize anything.it shows a black screen.

the issue is to kill the process on my phone and relaunch my appli.then every thing works fine.

how can I avoid this manipulation for my appli to work as soon as its wake

thanks for your help