ALV Tree and set_table_for_first_display problem

Question

I am trying to create an alv tree.  I have some sample code that uses the sflight table and this works fine however when i replace sflight with my internal structure and table it crashes at set_table_for_first_display with a "Field symbol has not been assigned" error.
This is part of the original code that works.  *This isn't all the code just the relevant parts.
DATA: gt_sflight      TYPE sflight OCCURS 0,   " Output-Table
           gt_fieldcatalog TYPE lvc_t_fcat,            " Field Catalog
CALL FUNCTION 'LVC_FIELDCATALOG_MERGE'
    EXPORTING
      i_structure_name = 'sflight'
    CHANGING
      ct_fieldcat      = gt_fieldcatalog.
*  change fieldcatalog
  DATA: ls_fieldcatalog TYPE lvc_s_fcat.
  LOOP AT gt_fieldcatalog INTO ls_fieldcatalog.
    CASE ls_fieldcatalog-fieldname.
      WHEN 'CARRID' OR 'CONNID' OR 'FLDATE'.
        ls_fieldcatalog-no_out = 'X'.
        ls_fieldcatalog-key    = ''.
      WHEN 'PRICE' OR 'SEATSOCC' OR 'SEATSMAX' OR 'PAYMENTSUM'.
        ls_fieldcatalog-do_sum = 'X'.
    ENDCASE.
    MODIFY gt_fieldcatalog FROM ls_fieldcatalog.
  ENDLOOP.
SELECT * FROM sflight INTO TABLE gt_sflight.
create hierarchy
  CALL METHOD tree1->set_table_for_first_display
    EXPORTING
      it_list_commentary = lt_list_commentary
      i_logo             = l_logo
      i_background_id    = 'ALV_BACKGROUND'
      i_save             = 'A'
      is_variant         = ls_variant
    CHANGING
      it_sort            = gt_sort
      it_outtab          = gt_sflight
      it_fieldcatalog    = gt_fieldcatalog.
Here is the code when I replace gt_sflight with my internal table.
DATA: gt_detailed      TYPE ZDT_REPORT1_DETAIL_STRUC occurs 0 ,   " Output-Table
           gt_fieldcatalog TYPE lvc_t_fcat,         " Field Catalog
CALL FUNCTION 'LVC_FIELDCATALOG_MERGE'
    EXPORTING
      i_structure_name = 'ZDT_REPORT1_DETAIL_STRUC'
    CHANGING
      ct_fieldcat      = gt_fieldcatalog
    EXCEPTIONS
      inconsistent_interface = 1
      program_error = 2
      OTHERS = 3.
    IF sy-subrc <> 0.
*--Exception handling
    ENDIF.
"* change fieldcatalog
  DATA: ls_fieldcatalog TYPE lvc_s_fcat.
  LOOP AT gt_fieldcatalog INTO ls_fieldcatalog.
    CASE ls_fieldcatalog-fieldname.
      WHEN 'COMP_DESC' OR 'SUBC_DESC'.
        ls_fieldcatalog-no_out = 'X'.
        ls_fieldcatalog-key    = ''.
      WHEN 'DOWNTIME'.
        ls_fieldcatalog-do_sum = 'X'.
    ENDCASE.
    MODIFY gt_fieldcatalog FROM ls_fieldcatalog.
  ENDLOOP.
" Select statement to fill gt_detailed
* create hierarchy
  CALL METHOD tree1->set_table_for_first_display
    EXPORTING
      it_list_commentary = lt_list_commentary
      i_logo             = l_logo
      i_background_id    = 'ALV_BACKGROUND'
      i_save             = 'A'
      is_variant         = ls_variant
    CHANGING
      it_sort            = gt_sort
      it_outtab          = gt_detailed
      it_fieldcatalog    = gt_fieldcatalog.
When it runs like this it crashes at set_table_for_first_display

Answer 1

The code I am using as an example is BCALV_TREE_SIMPLE_DEMO.  If I have no data in my table it doesn't error when I call set_table_for_first_display.
But it looks like I might have to use that one instead because from what I understand you can use  cl_gui_alv_tree_simple when the input table has all the required columns such as sflight but you use cl_gui_alv_tree when you need an internal table.