customEmbedHandlers not finished and does not produce an error either.

[madoka3530]

Even when set as shown below, the custom key cannot be converted.
The process does not finish and does not produce an error either.

 final dtm = DeltaToMarkdown(customEmbedHandlers: {
      'mention': (embed, out) {
        out.writeln('[mention]');
      },
    });
final markdown = dtm.convert(delta);

delta sample

[{insert: {mention: mention}},]

markdown_quill: ^3.1.0
flutter_quill: ^9.3.3