web dynpro alv row deselect

Hi,

is it possiple to deselect a row in an alv grid?

i try something like this

data node TYPE REF TO if_wd_context_node.
   data lv_old_index type i.
   data lv_new_index type i.
DATA: r_param TYPE REF TO if_salv_wd_table_lead_select.
   lv_new_index = r_param->index.
   lv_old_index = r_param->old_index.

IF lv_old_index = lv_new_index.
node = wd_context->get_child_node( 'ZPROFESSOR_AENDERN' ).
node->set_lead_selection_index( if_wd_context_node=>no_selection ).
ENDIF.

But i got this Error
Zugriff über 'NULL' Objektreferenz nicht möglich

my connected context node for the alv is ZPROFESSOR_AENDERN and it got the Cardinalität 0..N and the Selection 0..N

I realy tried a lot and search the web for a Solution but not a chance.

Please Help me .

Regards

TK