Using FileReader with React and Typescript

I want to upload json file using input with type file. When I use onload method on fileReder Typescript shows me error - Cannot invoke an object which is possibly 'null'.Thank you.

const UploadCharacter = () => {    const [data, setData] = useState<any>();    const handleUpload = (e: React.ChangeEvent<HTMLInputElement>) => {        if (e.target.files) {            const fileReader = new FileReader();             fileReader.readAsText(e.target.files[0], 'UTF-8')            fileReader.onload((e) => {  <-- Error: Cannot invoke an object which is possibly 'null'.                console.log(e.target.result)            })        }    }    return (<div className={s.upload}><input                className={s.uploadInput}                type="file"                onChange={handleUpload}            /></div>    )}