Fix possible nullptr deref

src/script/lua_api/l_object.cpp

@@ -690,7 +690,7 @@ int ObjectRef::l_set_observers(lua_State *L)
 	// Reset object to "unmanaged" (sent to everyone)?
 	if (lua_isnoneornil(L, 2)) {
 		ServerActiveObject *parent = sao->getParent();
-		if (!parent->m_observer_names.has_value())
+		if (parent != nullptr && !parent->m_observer_names.has_value())
 			throw LuaError("Child observers need to be managed if parent observers are managed");
 		sao->m_observer_names.reset();
 		return 0;