React-Native TypeError is not a function (evaluating_firebase

Good afternoon I have the following problem with an apk in react-native. I already try to find the error but I can not find any I hope you can help me I leave the code below.

ArtistDetailView.js

import React, { Component } from 'react';
import {
  Platform,
  StyleSheet,
  View,
  TextInput,
  TouchableOpacity
} from 'react-native';

import ArtistBox from './ArtistBox';
import getArtists from './api-client';
import firebaseDatabase from './firebase';
import CommentList from './CommentList';

import Icon from 'react-native-vector-icons/MaterialIcons';

export default class ArtistDetailView extends Component<Props> {

  state = {
    comments:[]
  }

  componentDidMount(){
    this.getArtistCommentsRef().on('child_added', (data)  =>{
      const comment = data.val()
      this.setState({
        comments: this.state.comments.concat(comment)
      })
    });
  }

  handleSend=()=>{
    const {text} = this.state
    const artistCommentsRef = this.getArtistCommentsRef()
    var newCommentRef = artistCommentsRef.push();
    newCommentRef.set({text});
  }

  getArtistCommentsRef = () => {
    const {id}= this.props.artist
    return firebaseDatabase.ref("comments/" + id)
  }

  handleChangeText = (text) => this.setState({text})

  render() {
    const artist = this.props.artist
    const {comments} = this.state
    return (
      <View style={styles.container}>
        <ArtistBox artist={artist}/>
        <CommentList comments={comments}/>
        <View style={styles.inputContainer}>
          <TextInput
          style={styles.input}
          placeholder="Nuevo Comentario"
          onChangeText={this.handleChangeText}
          />
          <TouchableOpacity onPress={this.handleSend}>
            <Icon name="send" size={25} color="lightblue"/>
          </TouchableOpacity>
        </View>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'white',
    paddingTop:10,

  },
  inputContainer:{
    position:'absolute',
    bottom:0,
    right:0,
    left:0,
    height:55,
    paddingHorizontal:10,
    backgroundColor:'#EBEBEC',
    flexDirection:'row',
    alignItems:'center',

  },
  input:{
    flex:1,
    height:55,
  }

});

CommentList.js

import React, { Component } from 'react';
import {
  Platform,
  StyleSheet,
  ListView

} from 'react-native';
import Comment from './Comment';


export default class CommentList extends Component<Props> {
  constructor(props) {
    super(props);
    const ds = new ListView.DataSource({rowHasChanged: (r1, r2) => r1 !== r2});
    this.state ={
      dataSource:ds
    }

  }
  componentDidMount(){
    this.updateDataSource(this.props.comments)
  }

  componentWillReceiveProps(newProps){
    if (newProps.comments !== this.props.comments){
      this.updateDataSource(newProps.comments)
    }
  }

  updateDataSource = (data)=>{
    this.setState({
      dataSource: this.state.dataSource.cloneWithRows(data)
    })
  }


  render() {

    return (
      <ListView
        enableEmptySections ={true}
        style={styles.container}
        dataSource={this.state.dataSource}
        renderRow={(comment) => {
          return(
            <Comment text={comment.text}/>
          )
        }}
      />
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'white',
    paddingTop:10,
  },

});

Comment.js

    import React from 'react';
    import {
      Text

    } from 'react-native';

    const Comment = (props) => 
        <Text>{props.text}</Text>

API-CLIENT.JS

const URL='http://ws.audioscrobbler.com/2.0/?method=geo.gettopartists&country=mexico&api_key=af0a08ae2fa637dea7fa0a2ce0a977b4&format=json'

function getArtists() {
  return fetch(URL)
    .then(response => response.json())
    .then(data => data.topartists.artist)
    .then(artists => artists.map(artist => {
      return{
        id: artist.mbid,
        name: artist.name,
        image: artist.image[3]['#text'],
        likes:200,
        comments:140,
      }
    }))

}

export default getArtists