Fixup thread-owned lambda bookkeeping on thread exit

modules/gdscript/gdscript_lambda_callable.cpp

@@ -296,5 +296,7 @@ GDScriptLambdaSelfCallable::GDScriptLambdaSelfCallable(Object *p_self, GDScriptF
 }
 
 GDScriptLambdaSelfCallable::~GDScriptLambdaSelfCallable() {
-	GDScript::_remove_func_ptr_to_update(updatable_func_ptr_element);
+	if (updatable_func_ptr_element) {
+		GDScript::_remove_func_ptr_to_update(updatable_func_ptr_element);
+	}
 }