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Why I get error "Cannot read property 'state' of undefied in react-native? [duplicate]

I very need your help. I am trying to make a JSON object users in my state properties to check how my authentification works. But when I tried to get access I had received the error "Cannot read property 'state' of undefied" and error arrow pointed on this part of code const { users, textInputEmail, textInputPassword } = this.state. Also, when I tried to check users it also showed 'undefined'. What have I done wrong?

import React, { Component } from 'react'
import { View, TextInput } from 'react-native'
import { MyButton, ErrorMessage } from '../uikit'
import { FormStyle, InputStyle } from '../constants/styles'
import { SIGN_IN, SUCCESS } from '../routes'

export class LogIn extends Component {

    state = {
        users: {
            user: [
                {
                    name: 'john1303@ukr.net',
                    password: '12345678'
                },

                {
                    name: 'steve13@gmail.com',
                    password: '87654321'
                }

            ],
        },
        textInputEmail: '',
        textInputPassword: ''
    }

    isUser() {
        console.log(users)
        const { users, textInputEmail, textInputPassword } = this.state
        let currentUser, password;

        currentUser = users.map((user) => user.name == textInputEmail ? user : 'unknown')

        if (currentUser == 'unknown')
            return alert('Incorrect user or password!')
        else {
            if (currentUser.password == textInputPassword)
                this.props.navigation.navigate(SUCCESS)
        }
    }

    render() {
        const { mainContainer, buttons } = FormStyle
        const { container, text } = InputStyle

        return (
            <View>
                <View style={mainContainer}>
                    <View style={container}>
                        <TextInput
                            style={text}
                            placeholder={'Email/Login'}
                            onChangeText={(value) => this.setState({ textInputEmail: value })}
                        >
                        </TextInput>
                        <ErrorMessage errorText={'Incorrect email'} />
                    </View>

                    <View style={container}>
                        <TextInput
                            style={text}
                            placeholder={'Password'}
                            secureTextEntry={true}
                            onChangeText={(value) => this.setState({ textInputPassword: value })}
                        >
                        </TextInput>
                        <ErrorMessage errorText={'Incorrect password'} />
                    </View>

                    <View style={buttons}>
                        <MyButton
                            name={'Log in'.toUpperCase()}
                            onPress={this.isUser} />
                        <MyButton
                            name={'Sign in'.toUpperCase()}
                            onPress={() => this.props.navigation.navigate(SIGN_IN)} />
                    </View>
                </View>
            </View>
        )
    }
}
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