Getting [NoErrorObjectAvailable] Script error.

Its showing toast message but also showing this error box always ,I cant get why its showing 

import { LightningElement, track } from 'lwc';
import { subscribe, unsubscribe, onError } from 'lightning/empApi';
import { ShowToastEvent } from 'lightning/platformShowToastEvent';
export default class SubscribeToPlatformEvent extends LightningElement {
    @track showMessage = false;
    @track message = '';
    channelName = '/event/Tata_Tele_Services__e';
    isComponentConnected = true;
    connectedCallback() {
        onError(error => {
            console.error('EMP API error: ', JSON.stringify(error));
        });
    }
    subscribeToPlatformEvent() {
        try {
            const messageCallback = response => {
                console.log('Message received:', response);
                try {
                    if (!this.isComponentConnected) {
                        // Component is disconnected, no need to process the event
                        return;
                    }
                    const agentNumber = response.data.payload.Agent_Number__c
                    // Display a toast message for every incoming call
                    const toastEvent = new ShowToastEvent({
                        title: 'Incoming Call',
                        message: `Incoming call from ${agentNumber}`,
                        variant: 'info',
                    });
                    console.log('Successfully subscribed to : ', toastEvent);
                    if (this.isComponentConnected) {
                        this.dispatchEvent(toastEvent);
                    }
                    console.log('Successfully subscribed to : ', this.dispatchEvent);
                } catch (error) {
                    console.error('Error handling message: ', JSON.stringify(error));
                }
            };
            // Subscribe to the Platform Event
            subscribe(this.channelName, -1, messageCallback).then(response => {
                console.log('Successfully subscribed to : ', JSON.stringify(response));
            });
        } catch (error) {
            console.error('Error subscribing to platform event: ', JSON.stringify(error));
        }
    }
    // Unsubscribe when the component is destroyed
    disconnectedCallback() {
        this.isComponentConnected = false;
        unsubscribe(this.channelName);
    }
}
HTML
<template>
    <lightning-card title="Incoming Call">
        <div class="slds-m-around_medium">
            <!-- Remove "utility:" prefix from icon-name -->
            <lightning-icon icon-name="down" size="small"></lightning-icon>
            <lightning-button label="Subscribe" onclick={subscribeToPlatformEvent}></lightning-button>
            <div if:true={showMessage}>
                <p>{message}</p>
            </div>
        </div>
    </lightning-card>
</template>

#Integration  #Platform-events  #LWC  #LWC Framework