我的笑话+酶mount()测试遇到问题。我正在测试一个功能，该功能可以切换显示组件。

组件之间的开关：当state.infoDisplayContent = 'mission'一个missionControl部件被安装时，当state.infoDisplayContent = 'profile'-其它组分的步骤中：

_modifyAgentStatus () {
    const { currentAgentProfile, agentsDatabase } = this.state;
    const agentToMod = currentAgentProfile;

    if (agentToMod.status === 'Free') {
        this.setState({
            infoDisplayContent: 'mission'
        });
        agentToMod.status = 'Waiting';
    } else if (agentToMod.status === 'Waiting') {
        const locationSelect = document.getElementById('missionLocationSelect');

        agentToMod.location = locationSelect[locationSelect.selectedIndex].innerText;
        agentToMod.status = 'On Mission';
        this.setState({
            infoDisplayContent: 'profile'
        });
    }
}

当我触发此函数时，一切看起来都很好，该测试运行良好，并且测试成功通过了所需的组件：

    import React from 'react';
    import { mount } from 'enzyme';
    import App from '../containers/App';

    const result = mount(
        <App />
    )

    test('change mission controls', () => {
        expect(result.state().currentAgentProfile.status).toBe('Free');
        result.find('#statusController').simulate('click');
        expect(result.find('#missionControls')).toHaveLength(1);
        expect(result.find('#missionLocationSelect')).toHaveLength(1);
        expect(result.state().currentAgentProfile.status).toBe('Waiting');
    });

    But when I simulate onClick two times: 

    test('change mission controls', () => {
        expect(result.state().currentAgentProfile.status).toBe('Free');
        result.find('#statusController').simulate('click');
        expect(result.find('#missionControls')).toHaveLength(1);
        expect(result.find('#missionLocationSelect')).toHaveLength(1);
        expect(result.state().currentAgentProfile.status).toBe('Waiting');
        result.find('#statusController').simulate('click');
        expect(result.state().currentAgentProfile.status).toBe('On Mission');
    });

我得到这个断言：

        TypeError: Cannot read property 'selectedIndex' of null

      at App._modifyAgentStatus (development/containers/App/index.js:251:68)
      at Object.invokeGuardedCallback [as invokeGuardedCallbackWithCatch] (node_modules/react-dom/lib/ReactErrorUtils.js:26:5)
      at executeDispatch (node_modules/react-dom/lib/EventPluginUtils.js:83:21)
      at Object.executeDispatchesInOrder (node_modules/react-dom/lib/EventPluginUtils.js:108:5)
      at executeDispatchesAndRelease (node_modules/react-dom/lib/EventPluginHub.js:43:22)
      at executeDispatchesAndReleaseSimulated (node_modules/react-dom/lib/EventPluginHub.js:51:10)
      at forEachAccumulated (node_modules/react-dom/lib/forEachAccumulated.js:26:8)
      at Object.processEventQueue (node_modules/react-dom/lib/EventPluginHub.js:255:7)
      at node_modules/react-dom/lib/ReactTestUtils.js:350:22
      at ReactDefaultBatchingStrategyTransaction.perform (node_modules/react-dom/lib/Transaction.js:140:20)
      at Object.batchedUpdates (node_modules/react-dom/lib/ReactDefaultBatchingStrategy.js:62:26)
      at Object.batchedUpdates (node_modules/react-dom/lib/ReactUpdates.js:97:27)
      at node_modules/react-dom/lib/ReactTestUtils.js:348:18
      at ReactWrapper.<anonymous> (node_modules/enzyme/build/ReactWrapper.js:776:11)
      at ReactWrapper.single (node_modules/enzyme/build/ReactWrapper.js:1421:25)
      at ReactWrapper.simulate (node_modules/enzyme/build/ReactWrapper.js:769:14)
      at Object.<anonymous> (development/tests/AgentProfile.test.js:26:38)
      at process._tickCallback (internal/process/next_tick.js:109:7)

显而易见的是：

document.getElementById('missionLocationSelect');

返回null，但我无法理解为什么。正如我提到的那样，元素通过了测试。

expect(result.find('#missionLocationSelect')).toHaveLength(1);

但无法使用捕获document.getElementById()。

请帮助我解决此问题并运行测试。