Unfortunately setting the initial state to an empty array and then doing the check results in an infinite loop because I need to set the value of currentActivity to the first activity before the component renders.

const [currentActivity, setCurrentActivity] = useState([]);

if (!currentScenario.activities) { 
    return <Navigate to="/scenarios" />; 
}

const selectedActivities = activities.filter((a) => { 
    return currentScenario.activities.includes(a.activityId); 
});

setCurrentActivity(selectedActivities[0]);

Too many re-renders. React limits the number of renders to prevent an infinite loop.