Ok, that line of code was intended for Joom!Fish on Joomla 1.0 (or 1.5 legacy), but it seems as though a property with the same name has re-surfaced in Joomla 1.7 but it is protected. Please try amending the /administrator/components/com_nbill/framework/interop/nbill.interop.abstract.php file, line 487 and insert an @ symbol at the start of the line, so lines 485 to 488 look like this:
if ($mainframe)
{
@$mainframe->_clientId = 0; //Fool Joom!Fish into thinking it is running in the front end
}
You will need to do the same thing on line 527:
@$mainframe->_clientId = $hold_j_client_id;