Reactjs React本机呈现程序中的Ref为null。单元测试中的create（）调用未通过我的测试

我正在使用

"react": "^16.4.0",
"jest": "^22.4.4",

然后在我的组件中，我使用react native下拉警报

 import DropdownAlert from 'react-native-dropdownalert';

在我的渲染函数中，我把它放在了最底部

  <DropdownAlert
      ref={this.dropdownRef}
    />

然后，我在渲染之前用这一行触发下拉列表。这是导致单元测试中出现问题的那一行

this.dropdownRef.current.alertWithType('error', 'Error', this.props.firebaseInfo.errorMessage);

这是我的单元测试

import 'react-native';
import React from 'react';
import renderer from 'react-test-renderer';
import { SignUp } from '../SignUp';
import Enzyme, { mount, shallow } from 'enzyme'


function createNodeMock() {

  return {
    DropdownAlert: {
      alertWithType() {

      }
    },
    current: {
      DropdownAlert: {
        alertWithType() {}
      }
    },
    alertWithType() {}
  }
}


describe('Sign up page component', () => {
  it('sign up renders correctly', () => {

    const props = {
      actions: {},
      userInfo: {},
      firebaseInfo: {},
    };

    const tree = renderer.create(<SignUp {...props}/>, {createNodeMock}).toJSON();

    expect(tree).toMatchSnapshot();
  });
});

import 'react-native';
import React from 'react';
import renderer from 'react-test-renderer';
import { SignUp } from '../SignUp';
import Enzyme, { mount, shallow } from 'enzyme'


function createNodeMock() {

  return {
    DropdownAlert: {
      alertWithType() {

      }
    },
    current: {
      DropdownAlert: {
        alertWithType() {}
      }
    },
    alertWithType() {}
  }
}


describe('Sign up page component', () => {
  it('sign up renders correctly', () => {

    const props = {
      actions: {},
      userInfo: {},
      firebaseInfo: {},
    };

    const tree = renderer.create(<SignUp {...props}/>, {createNodeMock}).toJSON();

    expect(tree).toMatchSnapshot();
  });
});

TypeError: Cannot read property 'alertWithType' of null