//trigger when-walidate-item :
i number;
vTimer TIMER;
begin

IF //condition then
i := show_alert('ERROR');
/* Create a timer with a 10 Millisecond delay */
vTimer :=create_timer('TEMP',10,no_repeat);

end IF;

end;

//trigger when-timer-expired
GO_BLOCK ('name_block');

:name_block.item1:=NULL;

DELETE_TIMER('TEMP');

But in runtime ,i was this error :
FRM-40202 Field must be entered !!

declare
  l_exists varchar2(1);
begin
  select null
    into l_exists
    from your_table t
    where t.id = :block.item_name;

  message('Such a value already exists in the database');
  :block.item_name := null;
  raise form_trigger_failure;

exception
  when no_data_found then
    -- OK, no problem - it doesn't exist
    null;
end;

		if condition1 then
				i := show_alert('ERROR_CODE_GR');
							 :block.item1:=null;
							   raise form_trigger_failure;
end if;

							 
//condition NOT NULL
	if :block.item1 is null then
			j:=show_alert('err2');
			raise form_trigger_failure;
		end if;
	
exception
  when no_data_found then
    -- OK, no problem - it doesn't exist
    null;
		end;

		if condition1 then
				i := show_alert('ERROR_CODE_GR');
							 :block.item1:=null;
							   raise form_trigger_failure;
end if;

							 
//condition NOT NULL
	if :block.item1 is null then
			j:=show_alert('err2');
			raise form_trigger_failure;
		end if;
	
exception
  when no_data_found then
    -- OK, no problem - it doesn't exist
    null;
		end;