KEYCLOAK-2700 Import existing realm fails due to can't delete group

model/jpa/src/main/java/org/keycloak/models/jpa/JpaRealmProvider.java

@@ -127,6 +127,10 @@ public class JpaRealmProvider implements RealmProvider {
         em.refresh(realm);
         RealmAdapter adapter = new RealmAdapter(session, em, realm);
         session.users().preRemove(adapter);
+
+        realm.getDefaultGroups().clear();
+        em.flush();
+
         int num = em.createNamedQuery("deleteGroupRoleMappingsByRealm")
                 .setParameter("realm", realm).executeUpdate();
         num = em.createNamedQuery("deleteGroupAttributesByRealm")