Know the References Between Analytic Applications and Custom Widgets
Exported archives of analytic applications by default contain all referenced custom widgets.
If you use a custom widget in an analytic application and the application is transported between different SAP Analytics Cloud systems, the application archive automatically includes the referenced custom widget (as it works for models as well). In this way only one archive needs to be imported to the target system to get the application up and running.

If you choose to ignore the warning and delete the custom widget regardless of active use and later realize that this was a mistake and re-upload the custom widget, the references between applications and this custom widget will stay lost.
This means that the export of the custom widget with the analytic application actively using it and the display of related applications in the dialog won't work anymore. In this case, you'll see a warning when opening an affected application:

To solve this, just save the application. You can therefore repair the dependencies and will no longer see the warning.