runtime error for ui element multipane

hello.

i have a question.

i implemented uielement multipane example in wda.

it worked very well in only first trying, but it occured a runtime error -that is Access via 'NULL' object reference not possible.- from second trying.

context -

hd <node>

-

attribute1 <hd of attribute>

-

attribute2 <hd of attribute>

-

item <hd of subnode>

-

attribute1 <item of attribute>

-

attribute2 <item of attribute>

below is source.

==========================================================

DATA: node_hd type ref to if_wd_context_node,

node_item type ref to if_wd_context_node,

itab_hd type wd_this->elements_hd,

stru_hd type wd_this->element_hd,

itab_item type wd_this->elements_item,

rt_auart type ref to data,

rt_vbeln type ref to data.

field-symbols: <fs_auart> type table,

<fs_vbeln> type table.

• retrieve the data from the select option

rt_auart = wd_this->m_handler->get_range_table_of_sel_field( i_id = 'AUART' ).

• assign it to a field symbol

assign rt_auart->* to <fs_auart>.

• retrieve the data from the select option

rt_vbeln = wd_this->m_handler->get_range_table_of_sel_field( i_id = 'VBELN' ).

• assign it to a field symbol

assign rt_vbeln->* to <fs_vbeln>.

• retrieve that data from the database.

• Normally it is suggested to encapsulate the data retrieval in a separate class.

• For simplicity, the select statement has been implemented here.

select * into corresponding fields of table itab_hd

from vbak

where vbeln in <fs_vbeln>

and auart in <fs_auart>.

• bind the data to the context

node_hd = wd_context->get_child_node( name = 'HD' ).

node_hd->bind_table( itab_hd ).

• bind the data to the context

loop at itab_hd into stru_hd.

• retrieve item_node object each sy-tabix

• i could't get item_node object when sy-tabix is 1.

• i hope to know why i can't get item_node object when sy-tabix is 1.

node_hd->get_child_node(

exporting

index = sy-tabix

name = 'ITEM'

receiving

child_node = node_item ).

***************************************************************************

• when index = 1

• node_item object is null.

*

• why node_item object is null?????

• i don't got it.

• can sombody explain about it???

• because of this matter, i can't acess frist node_item to insert value.

• do you have a solving this matter?

*

***************************************************************************

select * into corresponding fields of table itab_item

from vbap

where vbeln = stru_hd-vbeln.

node_item->bind_table( itab_item ).

endloop.