Docs-addon: Error displayed when viewing non-mdx stories

Describe the bug When leveraging the docs plugin (@storybook/addon-docs) to render MDX files on an Angular project that has pre-existing *.stories.ts files, an error is displayed when viewing the Docs add-on.

To Reproduce Steps to reproduce the behavior:

1. Navigate to Story that is written within *.stories.ts file.
2. Select “Canvas” Tab
3. Component renders properly
4. Select “Docs” Tab
5. See error
6. Select “Canvas” Tab
7. See error

Expected behavior Docs addon should be hidden or display a user friendly error if there is not documentation associated with the story. Error should not be displayed for subsequent use of storybook after seeing error.

Screenshots If applicable, add screenshots to help explain your problem.

Code snippets

System:

• OS: Windows 10
• Browser: Chrome, Edge, Firefox
• Framework: Angular
• Addons: @storybook/addon-docs
• Version: 5.2.0-alpha.41

{
   ...
    "@storybook/addon-a11y": "5.2.0-alpha.41",
    "@storybook/addon-actions": "5.2.0-alpha.41",
    "@storybook/addon-docs": "5.2.0-alpha.41",
    "@storybook/addon-knobs": "5.2.0-alpha.41",
    "@storybook/addon-links": "5.2.0-alpha.41",
    "@storybook/addon-notes": "5.2.0-alpha.41",
    "@storybook/addon-viewport": "5.2.0-alpha.41",
    "@storybook/addons": "5.2.0-alpha.41",
    "@storybook/angular": "5.2.0-alpha.41",
   ...
}

[domjacks]

So we have this:

import React from 'react';
import { storiesOf } from '@storybook/react';
import { text } from '@storybook/addon-knobs';
import Component from '.';
import readme from './README.md';

storiesOf('Component', module)
  .add(
    'default',
    () => {
      const content = text('content', 'Text Here');

      return <Component>{content}</Component>;
    },
    { notes: { markdown: readme }, component: Component }
  )
  .add(
    'second',
    () => {
      const content = text('content', 'Text Here');

      return <Component>{content}</Component>;
    },
    { notes: { markdown: readme }, component: Component }
  );

[domjacks]

Are you using notes or or info? Pretty sure it has something to do with that. Do you have a repro I could look at?

Unfortunately it’s a private repo. But we’re using notes. I’ve just enabled DocsPage in my config:

addParameters({
  docs: DocsPage
});

If there is only one story for the component, it renders the docs with the primary story along with the prop table (using prop-types and docgen) all correctly. As soon as I add a second story using .add() I get the error.