CIImage pixelBuffer always return nil - swift

Question

I am doing some task to apply filter effect in to my WebRTC call, follow this tutorial:
https://developer.apple.com/documentation/vision/applying_matte_effects_to_people_in_images_and_video
Here is my code to convert:
func capturer(_ capturer: RTCVideoCapturer, didCapture frame: RTCVideoFrame) {
let pixelBufferr = frame.buffer as! RTCCVPixelBuffer
let pixelBufferRef = pixelBufferr.pixelBuffer
if #available(iOS 15.0, *) {
DispatchQueue.global().async {
if let output = GreetingProcessor.shared.processVideoFrame(
foreground: pixelBufferRef,
background: self.vbImage) {
print("new output: \(output) => \(output.pixelBuffer) + \(self.buffer(from: output))")
guard let px = output.pixelBuffer else { return }
let rtcPixelBuffer = RTCCVPixelBuffer(pixelBuffer: px)
let i420buffer = rtcPixelBuffer.toI420()
let newFrame = RTCVideoFrame(buffer: i420buffer, rotation: frame.rotation, timeStampNs: frame.timeStampNs)
self.videoSource.capturer(capturer, didCapture: newFrame)
}
}
}
}
THen here is how I apply effect:
func blendImages(
background: CIImage,
foreground: CIImage,
mask: CIImage,
isRedMask: Bool = false
) -> CIImage? {
// scale mask
let maskScaleX = foreground.extent.width / mask.extent.width
let maskScaleY = foreground.extent.height / mask.extent.height
let maskScaled = mask.transformed(by: __CGAffineTransformMake(maskScaleX, 0, 0, maskScaleY, 0, 0))
// scale background
let backgroundScaleX = (foreground.extent.width / background.extent.width)
let backgroundScaleY = (foreground.extent.height / background.extent.height)
let backgroundScaled = background.transformed(
by: __CGAffineTransformMake(backgroundScaleX, 0, 0, backgroundScaleY, 0, 0))
let blendFilter = isRedMask ? CIFilter.blendWithRedMask() : CIFilter.blendWithMask()
blendFilter.inputImage = foreground
blendFilter.backgroundImage = backgroundScaled
blendFilter.maskImage = maskScaled
return blendFilter.outputImage
}
The problem is output.pixelBuffer always nil, so I can not create RTCFrame to pass it again to delegate
Can someone help?