JSON.stringify cannot serialize cyclic structure when passing state - reactjs

Question

I have problem when fetching data in react-native. I'm trying to send parts of my state in body of POST. I dont understand where is cyclic structure in my code. I dont see any cyclic structures.
I know the error is caused by state because when i simply put some strings in body everything is going alright.
import React from 'react';
import {Text, View, Button, StyleSheet, TextInput, TouchableOpacity} from "react-native";
export default class AddKidScreen extends React.Component {
constructor(props) {
super(props);
this.state = {
name: '',
age: '',
parentID: props.navigation.getParam('myID', 0)
}
}
handleAddKid() {
let url = "https://piotr2.scementowani.pl/apiPiotr";
let name = this.state.name;
let age = this.state.age;
fetch(url, {
method: 'POST',
headers: {
'Content-Type': "application/json",
},
body: JSON.stringify({
method: "addKid",
name: name,
age: age,
}),
})
.then(response => response.json())
.then(responseJson => {
this.props.navigation.navigate('Main');
})
.catch((error) => {
console.error(error);
});
}
updateValue(text,field) {
if (field == 'name') {
this.setState({
name: text,
})
} else if (field == 'age' ){
this.setState( {
age: text,
})
}
}
render() {
return(
<View>
<View style={{flexDirection: 'row', width: '100%'}}>
<Text>
Imię:
</Text>
<TextInput
placeholder = "Imię"
onChange = {(text) => this.updateValue(text,'name')}
/>
</View>
<View style={{flexDirection: 'row', width: '100%'}}>
<Text>
Wiek:
</Text>
<TextInput
placeholder = "Wiek"
onChange = {(text) => this.updateValue(text,'age')}
/>
</View>
<TouchableOpacity onPress={this.handleAddKid.bind(this)}>
<Text>DODAJ</Text>
</TouchableOpacity>
</View>
)
}
}
When running handleAddKid()
I get JSON.stringify cannot serialize cyclic structure.