def paidTxid(self):
        """only used when Request status is PR_PAID"""
        if not self._req:
            return ''
        is_paid, conf_needed, txids = self._wallet.wallet._is_onchain_invoice_paid(self._req)
        if len(txids) > 0:
            return txids[0]
        return ''
only used when Request status is PR_PAID
electrum/gui/qml/qerequestdetails.py