Runtime error from null value

Question

I have a request to create a Function Module that will be assigned to an Action Box, which when executed will copy long text for Quality Notification tasks.  I have the module created and working, but after my code is executed a runtime error happens due to a null value being passed to a method in the SAP program QM07_UPDATE_ACTIONBOX.  It looks to me like it is trying to perform some kind of cleanup tasks, and is trying to delete a node tree that does not exist.  My code did nothing with any kind of node tree, so I am not sure how to avoid this runtime error.  My code works as desired, except for this error that occurs after my code is processed.  Any assistance would be greatly appreciated!  Below are more details from the error received...
Error analysis
    An exception occurred that is explained in detail below.
    The exception, which is assigned to class 'CX_SY_REF_IS_INITIAL', was not caught in procedure "QM07_UPDATE_ACTIONBOX" "(FUNCTION)", nor was it propagated by a RAISING clause.  Since the caller of the procedure could not have anticipated that the exception would occur, the current program is terminated.
    The reason for the exception is:
    You attempted to use a 'NULL' object reference (points to 'nothing') access a component (variable: "TREE_ACTIONBOX").   An object reference must point to an object (an instance of a class) before it can be used to access components.  Either the reference was never set or it was set to 'NULL' using the CLEAR statement.
Information on where terminated
    Termination occurred in the ABAP program "SAPLQM07" - in "QM07_UPDATE_ACTIONBOX".  The main program was "SAPLIQS0 ".
    In the source code you have the termination point in line 39 of the (Include) program "LQM07U12".  The termination is caused because exception "CX_SY_REF_IS_INITIAL" occurred in procedure "QM07_UPDATE_ACTIONBOX" "(FUNCTION)", but it was neither handled locally nor declared in the RAISING clause of its signature.
    The procedure is in program "SAPLQM07 "; its source code begins in line 1 of the (Include program "LQM07U12 ".
   28 * bei Tree werden Knoten und Items gelöscht bei Table Control wird
   29 * nur g_control_tab neu aufgebaut
   30   if g_tree = c_x.
   31 *---- Löschen der bestehenden Knoten und Items
   32
   33 * Tabelle der Knotenschlüssel erzeugen
   34     loop at g_node_tab into l_node.
   35       append l_node-node_key to l_dnodes_tab.
   36     endloop.
   37
   38 * Items
>>>>>     call method tree_actionbox->delete_all_items_of_nodes
   40       EXPORTING
   41         node_key_table          = l_dnodes_tab
   42       EXCEPTIONS
   43         failed                  = 1
   44         cntl_system_error       = 2
   45         error_in_node_key_table = 3
   46         dp_error                = 4.

Answer 1

Yes, I have searched the notes.  I used the keywords suggested in the dump...
If the error occures in a non-modified SAP program, you may be able to find an interim solution in an SAP Note.
If you have access to SAP Notes, carry out a search with the following keywords:
"OBJECTS_OBJREF_NOT_ASSIGNED" "CX_SY_REF_IS_INITIAL"
"SAPLQM07" or "LQM07U12"
"QM07_UPDATE_ACTIONBOX"