You should post an issue here on GitHub, so that Ivo (the widget author) can investigate. Looks like possibly a case where a widget library is being imported but the path to that library is incorrect.
So you did not have the GoogleMapsCustomMarker widget in your project and never had? Can you tell a bit more about the scenario? I can not replicate this.
Two tips if you do use the widget: Clean the deployment directory and update to the newest version of my GoogleMapsCustomMarker widget. There should not be any dependency with a async file.
[EDIT] : you logged a bug on my widget on GitHub, but you are actually not using my widget as I can see from your other issue with the info bubble parameters. Please contact that developer. I will close the issue on GitHub. Good luck!