Upgrade to Gatsby v2

src/cms/cms.js

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+import CMS from 'netlify-cms';
+import PagePreview from './preview-templates/page-preview';
+import PostPreview from './preview-templates/post-preview';
+
+CMS.registerPreviewTemplate('page', PagePreview);
+CMS.registerPreviewTemplate('post', PostPreview);

src/cms/preview-templates/page-preview.js

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+import React from 'react';
+import { Page } from '../../components/Page';
+
+const PagePreview = ({ entry, widgetFor }) => {
+  const body = widgetFor(['body']);
+  const title = entry.getIn(['data', 'title']);
+
+  return (
+    <Page title={title}>
+      <div dangerouslySetInnerHTML={{ __html: body }} />
+    </Page>
+  );
+};
+
+export default PagePreview;

src/cms/preview-templates/post-preview.js

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+import React from 'react';
+import { Content } from '../../components/Post/Content';
+
+const PostPreview = ({ entry, widgetFor }) => {
+  const body = widgetFor(['body']);
+  const title = entry.getIn(['data', 'title']);
+
+  return (
+    <Content body={body} title={title} />
+  );
+};
+
+export default PostPreview;