Preview not working properly after upgrade to 2.1.5

Describe the bug

Previews using a react component returns undefined after upgrading from the following packages:

@sanity/base               2.1.2 → 2.1.5
@sanity/components         2.1.0 → 2.1.4
@sanity/core               2.1.2 → 2.1.4
@sanity/default-layout     2.1.3 → 2.1.5
@sanity/default-login      2.1.0 → 2.1.4
@sanity/desk-tool          2.1.3 → 2.1.5
@sanity/google-maps-input  2.1.3 → 2.1.5
@sanity/vision             2.1.0 → 2.1.4

To Reproduce

Steps to reproduce the behavior:

1. Create a fresh project using the cli.
2. Add the following to your schemas/schema.js.

// First, we must import the schema creator
import createSchema from "part:@sanity/base/schema-creator";

// Then import schema types from any plugins that might expose them
import schemaTypes from "all:part:@sanity/base/schema-type";

const supportedLanguages = [
  { id: "en", title: "English", isDefault: true },
  { id: "da", title: "Danish" },
  { id: "nb", title: "Norwegian" },
  { id: "sv", title: "Swedish" },
];

// Then we give our schema to the builder and provide the result to Sanity
export default createSchema({
  // We name our schema
  name: "default",
  // Then proceed to concatenate our document type
  // to the ones provided by any plugins that are installed
  types: schemaTypes.concat([
    {
      name: "localeString",
      type: "object",
      fieldsets: [
        {
          title: "Translations",
          name: "translations",
          options: { collapsible: true },
        },
      ],
      fields: [
        {
          name: "key",
          title: "Key",
          type: "string",
          validation: (Rule) =>
            Rule.lowercase().warning("All lower case for a key is prefered"),
        },
        ...supportedLanguages.map((lang) => ({
          title: lang.title,
          name: lang.id,
          type: "string",
          fieldset: lang.isDefault ? null : "translations",
        })),
      ],
      preview: {
        select: {
          title: "key",
          ...supportedLanguages.reduce(
            (acc, cur) => ({ ...acc, [cur.id]: cur.id }),
            {}
          ),
        },
        prepare: ({ title, ...lngs }) => {
          const missingLngs = Object.entries(lngs)
            .filter((lng) => !lng[1])
            .map((lng) => lng[0]);
          return {
            title: `${title} - ${lngs.en}`,
            subtitle:
              missingLngs.length > 0
                ? `Missing: ${missingLngs.join(", ")}`
                : "Everything present",
            media: <span>{missingLngs.length > 0 ? "🚨" : "✔️"}</span>,
          };
        },
      },
    },
    {
      title: "Translations",
      name: "namespaces",
      type: "document",
      fields: [
        {
          name: "namespace",
          title: "Namespace",
          type: "string",
          validation: (Rule) => Rule.required().lowercase(),
        },
        {
          title: "translations",
          name: "translations",
          type: "array",
          of: [{ type: "localeString" }],
          validation: (Rule) =>
            Rule.unique().error(
              "Should not have a duplicate translation in the same namespace"
            ),
        },
      ],
    },
  ]),
});

1. Start studio and create a translation document. Add a value to the namespace field and add a new translation to the translation array.
2. See that array field is returned as undefined instead of specified component.

Expected behavior Expected too see an array element with ✔️ or 🚨 along with a title and subtitle, as seen in the screenshot. Instead I get undefined and no icon. This worked on previous versions mentioned above.

Screenshots Expected behaviour

Error behaviour

Which versions of Sanity are you using?

Run sanity versions in the terminal and copy-paste the result here.

@sanity/cli             1.149.16 (latest: 2.1.4)
@sanity/base               2.1.5 (up to date)
@sanity/components         2.1.4 (up to date)
@sanity/core               2.1.4 (up to date)
@sanity/default-layout     2.1.5 (up to date)
@sanity/default-login      2.1.4 (up to date)
@sanity/desk-tool          2.1.5 (up to date)
@sanity/vision             2.1.4 (up to date)

What operating system are you using? wsl2 ubuntu

Which versions of Node.js / npm are you running?

6.14.5
v14.4.0

Additional context

Security issue? No

[Bjodol]

Thanks @bjoerge - Look fine here as well 👍

[Aratramba]

thanks @bjoerge - works fine now.