The RadioSwitchControlMigrator uses the ControlMigratorRegistery in order to migrate sub fields. Unfortunately the controlMigrationMap is initialized based on the ControlMigratorRegistery during RadioSwitchControlMigrator class construction and has a restricted migrator visibility.
Let say that we have the following module dependency:
Module UI
Module Form
Module Data
Module Custom --> defines a RadioSwitchControl in relation with a Custom control.

Module form initializes controlMigrationMap with the migrator defined in UI and Form module. The migration of the RadioSwitchControl in the Custom module will fail because the Custom control was not register at the time RadioSwitchControlMigrator was constructed.

RadioSwitchControlMigrator has to access ControlMigratorRegistery once the Migrator.migrate() method is called and not during construction.