Merge pull request #36613 from akien-mga/tab-container-add-child-fix

Signals: tab_changed now is emitted when it's on scene tree

scene/gui/tab_container.cpp

@@ -538,7 +538,7 @@ void TabContainer::add_child_notify(Node *p_child) {
 
  update();
  p_child->connect_compat("renamed", this, "_child_renamed_callback");
- if (first)
+ if (first && is_inside_tree())
  emit_signal("tab_changed", current);
 }