Don't send not loaded value for on_change

review919003
(transplanted from ce9893ef48f9b9dba34c3487ff4fcc8f0deb9d7d)

--HG--
extra : transplant_source : %CE%98%93%EFH%F9%B9%DB%A3L4%87%FFO%CC%8F%0D%EB%9D%7D
Set button insensitive when calling their callback

review900002
(transplanted from d21e4282542a5bf8a4a4beebc673591782befbf8)

--HG--
extra : transplant_source : %D2%1EB%82T%2A%5B%F8%A4%A4%BE%EB%C6sY%17%82%BE%FB%F8