Multisite Manager 2.1.5 and Relationships - PHP Error

I am building 5 websites using EE 2.6.61 amd MSM 2.1.5, one is a parent site, with 4 subsidary sites. Any news published on the subsidary sites is then displayed on the parent news page. Each news item has the usual fields, but also has a relationship field, this is used to select a predetermined author from a seperate channel. Each site has its own news channel and news templares

The displaying of the news item on the parent site is working correctly, apart from the relationship field, which doesn’t display any data and also displays a PHP error. See attached image.

If the relationship is with an indivdual site then there is no errors, can somebody see if there is a way of getting the relationship fields displaying correctly and the PHP error resolved.

Thanks