Hawtio:OSGi:Framework: no error message while setting empty Bundle Start Level

Current behaviour:
There is no error message, when you set the Initial Bundle Start Level to empty and save it. This value should not be empty. The null Bundle Start level is not saved, however, the message after saving shows, that configuration was updated.

Expected behaviour:
Error message shows, when you try to save Bundle Start Level to empty. The saving of this value is prohibited.