ARFoundation "black screen". It cannot access camera on IOS

Hi, this problem draws me crazy. The AR Foundation cannot access camera. I’ve searched online for a lot but didn’t find a solution of my problem. I’m using ARFoundation preview 6 and installed AR kit XR Plugin 2.1.2. When I’m testing on Iphone Xs with Unity Remote 5, it shows a black screen with no camera permission pop up but the UI shows on the screen. I have checked the require ARkit support and filled out the Camera Usage Description in the player settings.
157837-screen-shot-2020-04-27-at-111640-am.png


When I test, it can run on the phone with the black screen but give me an error: NullReferenceException: Object reference not set to an instance of an object.
This refers to a code: “var screenCenter = Camera.current.ViewportToScreenPoint(new Vector3(0.5f, 0.5f));”
Even I changed the code to have an instance of camera and assign the AR camera to it in the inspector, the error disappeared but the screen is still black.

Can anyone help me?

Also I got these warnings