Fix crash on tree select when item no longer exists after DOM rebuild

Guard tree.get_children() with tree.exists() so a stale item ID from
a COV or lazy-load rebuild does not raise TclError.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -629,7 +629,7 @@ def on_tree_select(event):
 
     if element is not None:
         components_to_render = [child for child in element if child.tag == "comp"]
-    else:
+    elif tree.exists(selected_id):
         top_level_ids = tree.get_children(selected_id)
         for t_id in top_level_ids:
             comp_elem = app_state.tree_element_map.get(t_id)