Runtime error for ui element multipane

Question

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.

Answer 1

Hi Regina.
So you say if it is not declared as singleton it is created for each element. But
when I declare my subnode with cardinality 0..1 I face the same problem.
This is what I do:
I bind the elements to the parent node. Afterwards I loop all elements of the parent
node and try to get the subnode with element->get_child_node( ). If cardinality of
the sub node is 0..1 or 0..n the reference is NULL. If cardinality is 1..1 or 1..n the
reference is not initial.
At what step does the frameowrk create the subnode then?
Cheers,
Sascha