Export not working if dialogs defined using UI6

If you try to export  a translation in the Content translation app, regardless of the chosen format (excel, xliff, cvs, ..), and if the dialogs are defined with the Magnolia 6 UI framework, you get an error: "Export of translation file failed"

This is not happening if the dialogs are defined using Magnolia 5 UI framework

Steps to reproduce

• Create a page in the pages app
• Add a dialog defined using UI6, for example:

  form:
    properties:
      text:
        $type: textField
        label: Body
        defaultValue: testing
        rows: 5
        maxLength: 300
  

• Go to the Content translation app, select the page and click on "Export translation"

Expected results
The file should be exported with no error
 
Actual results
The file is not exported and you get an error, find the full error attached.

ERROR tion.ui.action.TranslationFileDownloadDialogAction: An error occurred when executing the command downloadTranslationFile from catalog translation
java.lang.ClassCastException: info.magnolia.ui.dialog.ConfiguredFormDialogDefinition cannot be cast to info.magnolia.ui.dialog.definition.FormDialogDefinition