With the latest version of PushPlugin, you do not need to add any script references in index.html. Please, try removing the <script src="Plugins/PushPlugin/PushNotification.js"></script> line from your code and let us know if this resolves your issue.
You do not need the script include because the latest version of the PushPlugin has the plugin script file defined as a js-module. Plugman handles any script includes during the build process so you don't have to worry about it. You can read more about js-module here.
To help you while working with custom plugins in Icenium, you can check this section in our documentation. Also, you can use the PushPlugin as an integrated plugin and you can learn more about it here.
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