Blank popup when editing custom annotation in Sanity Studio

    "@sanity/base": "^2.31.0",
    "@sanity/core": "^2.31.0",
    "@sanity/default-layout": "^2.31.0",
    "@sanity/default-login": "^2.31.0",
    "@sanity/desk-tool": "^2.31.0",
    "@sanity/eslint-config-studio": "^2.0.0",
    "@sanity/vision": "^2.31.0",

{
      name: 'expSent',
      title: 'Example Sentence',
      type: 'array',
      of: [
        {
          type: 'block',
          styles: [],
          lists: [],
          marks: {
            decorators: [],
            annotations: [
              {
                name: 'targetWord',
                title: 'Target Word',
                type: 'object',
                blockEditor: {
                  icon: () => 'H',
                },
                fields: [
                  {
                    name: 'conjugation',
                    title: 'Conjugation',
                    type: 'string',
                    initialValue: 'base',
                    options: {
                      list: conjugations,
                    },
                  },
                ],
              },
            ],
          },
        },
      ],
      validation: Rule =>
        Rule.custom(contents => {
          /**
           * Check for undefined
           * @see {@link <https://www.sanity.io/docs/validation#091e10f957aa>}
           */
          if (typeof contents === 'undefined')
            return 'Example sentence required';

          // Check the example sentence has only a single line
          if (contents.length !== 1)
            return 'Example sentence should be in a single line';

          const content = contents[0];

          // Check the length
          const numOfWords = content.children.reduce(
            (sum, span) =&gt; sum + span.text.length,
            0
          );
          if (numOfWords &gt; MAX_EXP_SENT_CHAR)
            return `Example sentence should be less-than-${MAX_EXP_SENT_CHAR}-character long. Current character length is ${numOfWords} characters`;

          // Check that one target word (phrase) is selected
          const numOfMarks = content.children.reduce(
            (sum, span) =&gt; (span.marks.length === 0 ? sum : sum + 1),
            0
          );
          if (numOfMarks === 0)
            return 'Please select a target word in the example sentence';
          if (numOfMarks &gt; 1)
            return 'Only one consecutive-portion can be selected as target word';

          return true;
        }),
    },

├─┬ @sanity/base@2.31.0
│ └── @sanity/ui@0.37.19
├─┬ @sanity/default-layout@2.31.0
│ └── @sanity/ui@0.37.19 deduped
├─┬ @sanity/default-login@2.31.0
│ └── @sanity/ui@0.37.19 deduped
├─┬ @sanity/desk-tool@2.31.0
│ ├─┬ @sanity/field@2.31.0
│ │ └── @sanity/ui@0.37.19 deduped
│ ├─┬ @sanity/form-builder@2.31.0
│ │ └── @sanity/ui@0.37.19 deduped
│ └── @sanity/ui@0.37.19 deduped
└─┬ @sanity/vision@2.31.0
  └── @sanity/ui@0.37.19 deduped