When you try to add a camera by name like this:

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var cameraName:String = "WebCam";
var cam:Camera = Camera.getCamera(cameraName);

or like this

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var cam:Camera = Camera.getCamera(cameranames[0]);

both return cam = null

Solution:

You need to add camera index like String in order to work

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var cameraName:String = "WebCam";
var camera:Camera;
const imax:int = Camera.names.length;
var i:int = 0;
while (i < imax) {
    if (Camera.names[i] == cameraName) {
        camera = Camera.getCamera(String(i));
        break;
    }
    i++;
}

or use this function

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const camera:Camera = getCamera("webCam");
       
private static function getCamera($cameraName_:String):Camera {
    const imax:int = Camera.names.length;
    var i:int = 0;
    while (i < imax) {
        if (Camera.names[i] == $camName_) {
            return Camera.getCamera(String(i));
        }
        i++;
    }
    return null;
}

so now you will have your camera that you wanted:

thank to:
pooja