Android Kotlin ACTION_CREATE_DOCUMENT gives non-working URI [duplicate]

i am trying to export a SQLiteDatabase to a file. I am using ACTION_CREATE_DOCUMENT intent to get a file. It shows me the dialog, fills out everything correctly, and the file is created with zero size. I can see the files using the file manager. Yet when I check onActivityResult the Uri passed to this method looks broken. And I get IOExceptions (file not found) when trying to create a File using Uri.path

val intent = Intent(Intent.ACTION_CREATE_DOCUMENT).apply {
    addCategory(Intent.CATEGORY_DEFAULT)
    type = "application/x-sqlite3"
    putExtra(Intent.EXTRA_TITLE, "export.db")

    putExtra(DocumentsContract.EXTRA_INITIAL_URI, Environment.DIRECTORY_DOCUMENTS)
}
startActivityForResult(intent!!,4711)

I can see the proper filename, if I press save in the dialog, I end up with Activity.RESULT_OK. But the uri look like content://com.android.providers.downloads.documents/document/26 and uri.path is /document/26 and this gives me IO Exceptions.

override fun onActivityResult(requestCode: Int, resultCode: Int, resultData: Intent?) {
        super.onActivityResult(requestCode, resultCode, resultData)

        if (requestCode == 4711 && resultCode == Activity.RESULT_OK) {
            resultData?.data?.also { documentUri ->

                try {
                    val db = DBBackend(context!!)
                    val dbFile = File(db.getDatabaseFilename())

                    var exportFile = File(documentUri.path)
                    exportFile.createNewFile()
                    dbFile.copyTo(exportFile)
                }
                catch( error : Exception )
                {
                    Log.e("FileError", error.toString() )
                    Toast.makeText(context!!,"error: ${error.toString()}", Toast.LENGTH_LONG).show()
                }
            }
        }
    }

This makes no sense to me. What is the proper way to get a writable file in the Downloads folder?

Thanks for any hints