Adding gradient behind image returns empty image - swift

Question

I have a round avatar image with a transparent background. I want to create a new round image of the same size out of the initial image, with a gradient background behind it. So it looks like standing in sky instead of having a transparent background.
Since I will use this image as tabbaritem’s image, I couldn’t use uiview and edit it’s background layer.
And to make it reusable I wanted to create a UIImage extension.
Below is what I do:
extension UIImage {
func gradientImage() -> UIImage? {
let width = self.size.width
let height = self.size.height
UIGraphicsBeginImageContextWithOptions(size, false, 0)
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue)
let colorSpace = CGColorSpaceCreateDeviceRGB()
guard let bitmapContext = CGContext(data: nil,
width: Int(width),
height: Int(height),
bitsPerComponent: 8,
bytesPerRow: 0,
space: colorSpace,
bitmapInfo: bitmapInfo.rawValue) else { return nil }
let locations: [CGFloat] = [0.0, 1.0]
let top = R.color.duckDimDarkGrey()?.cgColor
let bottom = R.color.duckPencilDark()?.cgColor
let colors = [top, bottom] as CFArray
guard let gradient = CGGradient(colorsSpace: colorSpace, colors: colors, locations: locations) else {
return nil
}
bitmapContext.drawLinearGradient(gradient, start: CGPoint.zero, end: CGPoint(x: 0, y: size.height), options: CGGradientDrawingOptions())
guard let cgImage = UIGraphicsGetImageFromCurrentImageContext()?.cgImage else { return nil }
UIGraphicsEndImageContext()
let img = UIImage(cgImage: cgImage)
return img
}
}
Here is how I use it:
Let image1 = UIImage(named: “test.png”)
self.tabBar.items[3].image = image1.gradientImage()
However I am getting an empty image somehow.