DefaultThemeManager NullPointerException

Logging into the admin console after singleFile migration import results in “Error: an unexpected error has occured” and in the log:

07:22:53,557 ERROR [] (default task-4) Uncaught server error: java.lang.NullPointerException
at org.keycloak.theme.DefaultThemeManager.loadTheme(
at org.keycloak.theme.DefaultThemeManager.getTheme(

The login, account, admin console, and email themes in all realms are each configured to “keycloak”. Themes are not set on any clients.

This configuration entails keycloak 12.0.1, jre1.8.0_271, PostgreSQL 13

Exporting with:

Importing with:
-Dkeycloak.migration.action=import -Dkeycloak.migration.provider=singleFile -Dkeycloak.migration.file=migration.json -Dkeycloak.migration.strategy=OVERWRITE_EXISTING

The migration process has been working routinely in 11.0.3 and arose only after upgrading both the export and the import servers to 12.0.1.

Any suggestions would be greatly appreciated. Thank-you,