[Error Handling] 배상건 / Cannot read properties of null

> ### 어떤 에러인가요?
>
> ### 에러 메시지
> ```shell
> Uncaught TypeError: Cannot read properties of null (reading 'postData')
> at EditPost (EditPost.tsx:39:1)
> at renderWithHooks (react-dom.development.js:16175:1)
> at mountIndeterminateComponent (react-dom.development.js:20913:1)
> at beginWork (react-dom.development.js:22416:1)
> at HTMLUnknownElement.callCallback (react-dom.development.js:4161:1)
> at Object.invokeGuardedCallbackDev (react-dom.development.js:4210:1)
> at invokeGuardedCallback (react-dom.development.js:4274:1)
> at beginWork$1 (react-dom.development.js:27405:1)
> at performUnitOfWork (react-dom.development.js:26513:1)
> at workLoopSync (react-dom.development.js:26422:1)
> ```
>
> ### 에러 핸들링 방법
> * 핸들링 방법에 대해서 간단하게 기록해보세요.
>
> ```js
> // 코드가 들어가도 좋아요!
> ```
>
> ### 에러 핸들링을 위해 참고한 레퍼런스 링크
> 링크 https://velog.io/@baek1008/Error-Cannot-read-properties-of-null-%ED%95%B4%EA%B2%B0