我在oracle apex中创建了一个序列，并将该序列链接到应用程序端的行。但是，在master-detail表单上，当我尝试添加新的表单记录时，它不会自动增加到新的Roster_ID。我可能犯了什么错误？请帮帮忙
下面是我的代码：

CREATE TABLE ROSTER(
    Roster_ID NUMBER(5),
    Working_Hours NUMBER(2),
    Shift_Date DATE,
    Shift_End_Date DATE,
    Shift_ID NUMBER(5) REFERENCES Shift (Shift_ID),
    Ward_ID NUMBER(5) REFERENCES WARD (Ward_ID),
    Ward_Name VARCHAR2(25) REFERENCES WARD (Ward_Name),
    CONSTRAINT PK_ROSTER PRIMARY KEY (Roster_ID),
    CONSTRAINT CHECK_WORKING_HOURS CHECK (Working_Hours IN (6, 8, 12))
);

CREATE SEQUENCE Roster_seq START WITH 5 INCREMENT BY 1 NOCACHE NOCYCLE;

CREATE OR REPLACE TRIGGER Roster_T1
BEFORE
INSERT ON Roster
FOR EACH ROW
BEGIN
  IF :NEW.Roster_ID is NULL THEN 
    select Roster_seq.nextval into :NEW.Roster_ID from SYS.DUAL; 
  END IF; 
END;
/
COMMIT;
/