Unable to INSERT record but inserting. Forms 4.5 error through Apps 10.7

Question

Hi All,
I have written the following code on save button on form 4.5 which has been used from APPS. I created the master-detail relation also.
It suppose to insert a row into header and then into lines.
Its inserting the row into both the tables but getting the following error in betweeen.
===========
FRM-40508: ORACLE error: unable to INSERT record.
ORA-01400: cannot insert NULL into ("VFI"."VFI_PC_DEFINITION_LINES"."LINE_ID")
===========
I am thinking problem is some where in some default triigers or on insert triggers
but unable to trace it. CAN anybody through a light on this ?
================================================
PROCEDURE VFI_INVPCS IS
i          number;
v_item_id     number;
v_item_id1     number;
v_temp      varchar2;
v_user_id NUMBER := 1111; --fnd_global.user_id;
Lv_ChkFlag VARCHAR2(1);
Lv_UnChkFlag VARCHAR2(1);
ln_header_seq NUMBER;
ln_lines_seq NUMBER;
BEGIN
BEGIN
SELECT inventory_item_id
INTO v_item_id
FROM inv.mtl_system_items
WHERE organization_id = 103
and segment1 = :VFI_PC_DEFINITION_HEADER.parent_inventory_item_id;
SELECT VFI_PC_DEFINITION_HEADER_SEQ.nextval
INTO ln_header_seq
FROM DUAL;
SELECT VFI_PC_DEFINITION_LINES_SEQ.nextval
INTO ln_lines_seq
FROM DUAL;
INSERT INTO VFI_PC_DEFINITION_HEADER( HEADER_ID
,PARENT_INVENTORY_ITEM_ID
,START_DATE
,END_DATE
,CREATED_BY
,CREATION_DATE
,LAST_UPDATED_BY
,LAST_UPDATE_DATE)
VALUES ( ln_header_seq
     ,v_item_id
     ,:VFI_PC_DEFINITION_HEADER.start_Date
     ,:VFI_PC_DEFINITION_HEADER.end_date
,v_user_id
     ,sysdate
     ,v_user_id
     ,SYSDATE
COMMIT; -- header
GO_BLOCK('VFI_PC_DEFINITION_LINES');
LOOP
SELECT inventory_item_id
INTO v_item_id1
FROM inv.mtl_system_items
WHERE organization_id = 103
AND segment1 = :VFI_PC_DEFINITION_LINES.child_inventory_item_id;
First_record;
IF :VFI_PC_DEFINITION_LINES.child_inventory_item_id is not null then
IF :CFY_PTID_FLAG='Y' THEN
Lv_ChkFlag:=:CFY_PTID_FLAG;
INSERT INTO VFI_PC_DEFINITION_LINES( LINE_ID
,HEADER_ID
,PARENT_INVENTORY_ITEM_ID
,CHILD_INVENTORY_ITEM_ID
,CREATED_BY
,CREATION_DATE
,LAST_UPDATED_BY
,LAST_UPDATE_DATE
,CFY_PTID_FLAG)
VALUES ( ln_lines_seq
,ln_header_seq
,v_item_id
,v_item_id1
,v_user_id
,SYSDATE
,v_user_id
,SYSDATE
,Lv_ChkFlag
END IF;
END if;
NEXT_RECORD;
EXIT WHEN :SYSTEM.LAST_RECORD='TRUE';      
END LOOP;
message('After Insert Child');
message('After Insert Child');
COMMIT;
END;
================================================

Answer 1

not sure on the block structure of your forms.
one would question why you seem to be doing a lot of the default insert work on the press of a button (when-button-pressed).
also if you have a master-detail relationship in the form - why you don't let forms handle the master-detail insert.
seems like you have database database blocks but are almost coding as if they are non-base table blocks.