I am using the code below to reload a screen when each time it's in focus. This works fine except when I make changes to the file during development then I get the error shown below the code example. Not sure if this matters since I won't be saving the file while people are actually using the app. Can't see pushing this out into production though with this issue, any help would be appreciated.

this.focusListener = navigation.addListener('didFocus', () => {

    fetchData();

});

Error message when saving file in development:

TypeError: undefined is not an object (evaluating '_this.focusListener = navigation.addListener('didFocus', function () {
        fetchData();
      })')

My app completely blows up and I have to reload it. Using Expo if that matters.

Also shows this error:

Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.