Return a GrrMessage instance from the transaction log or None.

def Get(self):
    """Return a GrrMessage instance from the transaction log or None."""
    try:
      value, reg_type = winreg.QueryValueEx(_GetServiceKey(), "Transaction")
    except OSError:
      return

    if reg_type != winreg.REG_BINARY:
      return

    try:
      return rdf_flows.GrrMessage.FromSerializedString(value)
    except message.Error:
      return