send_modified must wait the current event to be finished to call get_value

If current event is not finished, in most cases the comparison between the two
get_value calls is always false.

review1016002
(transplanted from 6db60c1cd238d7947b2be211ef8296a7182e1ece)

--HG--
extra : transplant_source : m%B6%0C%1C%D28%D7%94%7B%2B%E2%11%EF%82%96%A7%18.%1E%CE
Set value before validate record in button click

issue3322
review1022002
(transplanted from 93fe795cf2b97437598e9155a77957c48e0a1ae0)

--HG--
extra : transplant_source : %93%FEy%5C%F2%B9t7Y%8E%91U%A7yW%C4%8E%0A%1A%E0
Test if widget child is still there before using it

issue2867
review1023002
(transplanted from 790ef8fa1ed1784e88d38846b8649493d0f7b078)

--HG--
extra : transplant_source : y%0E%F8%FA%1E%D1xN%88%D3%88F%B8d%94%93%D0%F7%B0x