Can anyone please help me, id dont know why i keep getting this error while updating a form in apex.

ORA-20505: Error in DML: p_rowid=1179950, p_alt_rowid=ID, p_rowid2=, p_alt_rowid2=. ORA-01403: no data found
Error Unable to process row of table COM_ADVERT_TB.
OK

I have a automatic DML process, and a get pk process with the following:

declare
function get_pk return varchar2
is
begin
for c1 in (select COM_ADVERT_SEQ.nextval next_val
from dual)
loop
return c1.next_val;
end loop;
end;
begin
:P5_ID := get_pk;
end;

my cancel button has a condition id is null, and my apply changes has a condition id is not null.

CAn anyone please help me?