Fix crash calling `_activate()` on ShaderGlobalsOverride out of tree

Fixes #45984.

scene/main/shader_globals_override.cpp

@@ -221,6 +221,7 @@ void ShaderGlobalsOverride::_get_property_list(List<PropertyInfo> *p_list) const
 }
 
 void ShaderGlobalsOverride::_activate() {
+	ERR_FAIL_NULL(get_tree());
 	List<Node *> nodes;
 	get_tree()->get_nodes_in_group(SceneStringNames::get_singleton()->shader_overrides_group_active, &nodes);
 	if (nodes.size() == 0) {