I just found out an elusive quirk in the Application Builder.
ORA-01400: cannot insert NULL into ("FLOWS_030000"."WWV_FLOW_INSTALL_SCRIPTS"."INSTALL_ID")
I documented the full page flow for reproducing the problem in this OTN forum thread, so i am not going to repeat myself, i assume you know what my fellow citizen Niccolò Paganini was used to say ... ;-)
One workaround for this problem is to not use the DHMTL menu (the nice cascading menu that pops up when you press on the black arrow on the right hand side of the Application Builder icon) but just click on the icon and go to the main Supporting Objects page and then clicking on the installation scripts from there.
I don't expect this problem to happen many times, from the tests I've made it only occurs when you are creating a brand new application from scratch and then you are trying to create a supporting object script right away.
See message translations for ORA-01400 and search additional resources.