Thank you Dong for pointing this out.
We have updated the document for the plug-in icon from "This file is optional" to "This file is required".
Sorry for the inconvenience this may have caused.
About "Plugin Icon icon.png" part in "Steps for Plug-in Development", it said that icon file is optional.
But I think the icon file is required, if we do not have icon file, error will occur when I packing the plugin.
Please confirm it, thank you.