Did you check this handy blog post
on porting a Cordova project to AppBuilder? It might be very helpful for transferring your work to AppBuilder.
When importing a custom plugin, make sure to follow this procedure
and keep in mind that custom plugins do not work in the simulator and in the companion app and usually throw an undefined error. To test custom plugins, you need to build an actual application package and run it on device.
If you are still experiencing issues, would you mind providing more details such as what is the AppBuilder client that you're using, what are your custom plugins, what is your workflow and where are you getting the undefined errors? Screenshots, a copy of your project, and any other details you consider related will be more than welcome.
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