Fix T83539: 'activate_init' fails with buttons that have popups

Activating buttons was running too early, before the popup was
positioned on the screen,
causing data-selectors to be positioned incorrectly.

As the early button activation isn't needed, remove this call.

Ensure this doesn't happen again with an assert.

source/blender/editors/interface/interface.c

@@ -988,6 +988,11 @@ bool UI_but_active_only(const bContext *C, ARegion *region, uiBlock *block, uiBu
  */
 bool UI_block_active_only_flagged_buttons(const bContext *C, ARegion *region, uiBlock *block)
 {
+
+  /* Running this command before end-block has run, means buttons that open menus
+   * wont have those menus correctly positioned, see T83539. */
+  BLI_assert(block->endblock != 0);
+
   bool done = false;
   LISTBASE_FOREACH (uiBut *, but, &block->buttons) {
     if (but->flag & UI_BUT_ACTIVATE_ON_INIT) {

source/blender/windowmanager/intern/wm_operators.c

@@ -1403,8 +1403,6 @@ static uiBlock *wm_block_dialog_create(bContext *C, ARegion *region, void *userD
   UI_block_bounds_set_popup(
       block, 6 * U.dpi_fac, (const int[2]){data->width / -2, data->height / 2});
 
-  UI_block_active_only_flagged_buttons(C, region, block);
-
   return block;
 }