无法读取小写react js

我使用 react 为我的数据构建过滤搜索。一切正常，但后来出现错误：

未捕获（承诺）类型错误：无法读取未定义的属性“toLowerCase”。

当我将状态搜索更新为 null 时，错误变为

'toLowerCase' 为 null。

但是当我把它作为一个空字符串时，错误变成了undefined.

就像我说的，一切正常，这个错误突然出现，我需要你的帮助

这是代码

var React = require('react');
var firebase = require('firebase');
var $ = require('jquery');
var axios = require('axios');
var notify = require('bootstrap-notify');
var {Link} = require('react-router');

var SuppliersList = React.createClass({

  getInitialState: function(event){
    return{
      search:'Nicolas',
      data: [],
    }
  },
  updateSearch : function(event) {
    //this.setState({search: event.target.value.substr(0, 20)});
  },
  componentDidMount: function(){
    firebase.database().ref('/suppliers/').once('value').then(function(snapshot) {
      var arr = Object.values(snapshot.val());
      this.setState({data: arr,activity:false});
    }.bind(this));
  },

  render: function () {
    var filteredSuppliers = this.state.data.filter(
      (item) => {
        return item.name.toLowerCase().indexOf(
          this.state.search.toLowerCase()) !== -1;
      }
    )

      var items = filteredSuppliers.map((item, i)=>
      <tr key={item.id}>
       <td>{item.name}</td>
       <td>{item.email}</td>
       <td>{item.phone}</td>

     <td><Link className="btn btn-primary" to={'/edit/'+item.id}>Edit</Link></td>
   <td><Link className="btn btn-danger" to={'/delete/'+item.id}>Delete</Link></td>
        <td><a href="#" className="btn btn-warning" role="button" data-title="johnny" data-id="1">Favourite</a></td>
     </tr>
      );
    return (
       <div>
         <div style={{marginLeft:'50%'}} className="loader"></div>
       <input type="text" value={this.state.search} style={{marginTop:20}} placeholder="search a supplier" onChange={this.updateSearch} className="form-control" id="usr"/>

       <table className="table table-striped table-hover table-users">
         <thead>
           <tr>
             <th>Name</th>
             <th>Email</th>
             <th>Phone</th>
             <th></th>
             <th></th>
           </tr>
         </thead>

         <tbody>
           {items}
         </tbody>
       </table>
</div>

    )},
});

var Suppliers = React.createClass({
  render: function(){
    return (
      <div>
        <nav className="navbar navbar-default navbar-fixed">
          <div className="container-fluid">
            <div className="navbar-header">

              <button type="button" className="navbar-toggle" data-toggle="collapse" data-target="#navigation-example-2">
                <a href="#/branch">
                  <i className="fa fa-store"></i>
                </a>
              </button>

              <button type="button" className="navbar-toggle" data-toggle="collapse" data-target="#navigation-example-2">
                <a href="#/delete">
                  <i className="fa fa-trash"></i>
                </a>
              </button>

              <button type="button" className="navbar-toggle" data-toggle="collapse" data-target="#navigation-example-2">
                <a href="#/edit">
                  <i className="fa fa-edit"></i>
                </a>
              </button>

              <button type="button" className="navbar-toggle" data-toggle="collapse" data-target="#navigation-example-2">
                <a href="#/add">
                  <i className="fa fa-plus"></i>
                </a>
              </button>

              <a className="navbar-brand" href="#">Suppliers</a>
            </div>
            <div className="collapse navbar-collapse">
              <ul className="nav navbar-nav navbar-left">

              </ul>
              <ul className="nav navbar-nav navbar-right">
                <li>
                   <a href="#/add">
                      <i className="fa fa-plus"></i>
                    </a>
                </li>

                <li>
                   <a href="#/delete">
                      <i className="fa fa-trash"></i>
                    </a>
                </li>

                <li>
                   <a href="#/edit">
                      <i className="fa fa-edit"></i>
                    </a>
                </li>

                <li onClick={this.refresh}>
                   <a>
                      <i className="fa fa-refresh"></i>
                    </a>
                </li>

                <li>
                   <a href="#/branch">
                      <i className="fa fa-map-marker"></i>
                    </a>
                </li>

              </ul>
            </div>
          </div>
        </nav>

        <table className="table table-hover table-striped">

            <SuppliersList/>

      </table>
      </div>
    )},

  refresh: function(){
    location.reload();
  }
});

  module.exports = Suppliers;