我正在嘗試基于兩個文本輸入字段中是否都存在任何字符來啟用/禁用表單按鈕，但是由于某種原因，狀態長度在我的條件下呈現了錯誤，盡管我登錄狀態時卻顯示了錯誤。錯誤：const isEnabled = this.state.subject.length > 0 && this.state.emails.length > 0; //Uncaught TypeError: Cannot read property 'length' of undefined完整代碼：import React from 'react';export default class EmailAnnotationForm extends React.Component {    constructor(props) {        super(props);        this.state = {            csrf: '',            subject: '',            emails: '',            comment: ''        }        this.handleInputChange = this.handleInputChange.bind(this);        this.handleFormSubmit = this.handleFormSubmit.bind(this);        this.handleClearForm = this.handleClearForm.bind(this);        this.input = React.createRef();    }    componentDidMount() {        console.log(this.state.subject.length) // renders correct value => 0        this.setState({subject: this.props.title });    }    handleInputChange(event) {        const target = event.target;        const value = target.type === 'checkbox' ? target.checked : target.value;        const name = target.name;        this.setState({            [name]: value        });    }    handleClearForm() {        this.setState({            csrf: '',            subject: '',            emails: '',            comment: ''        })    }    handleFormSubmit(event) {        var emailSubject;          {            this.state.subject ? emailSubject = this.state.subject : emailSubject = this.props.title        }; //        const body = {            subject: emailSubject,            emails: this.state.emails,            comment: this.state.comment        };