ERROR ITMS-90717 while uploading app

While I was uploading my app to Apple App Store using Xcode 9, the following error appear:

ERROR ITMS-90717: "Invalid App Store Icon. The App Store Icon in the asset catalog in "xx.app" can't be transparent nor contain an alpha channel".

To resolve this error, you need to

1. Remove the alpha (i.e., the transparency attribute) in the image file. This can be done easily.

1.1. Double click on your app icon and the image will appear Preview app.
1.2. Choose File \ Export.
1.3. Untick "Alpha" (as shown below).

1.4. Click Save.

So, you have done the first step.

2. Regenerate all the icon file in various dimension.

3. Replace all the icon files in your project.

4. Last step, update the Images.xcassts file by dragging all appropriate
 icon file.

To check if all icon files have been updated, open the following directory in Finder. Ensure that all icon files' date/time matches the date/time that was done in step 2.

    [your app] \ Images.xcassets \  AppIcon.appiconset

5. Build and upload your app.