I'm getting the following "Warning":

Warning: Invalid argument supplied for foreach() in /home/xxxxx/subdomains/master/administrator/components/com_multisites/helpers/helper.php on line 603

With the following configuration:

Multisite - 1.2.75
Patches - 1.2.77
Joomla - 2.5.1
php - 5.3.10

Note: When I install multisite 1.2.73 instead of 1.2.75 I do not get this warning message. I didn't download 1.2.74

Patches are applied. I'm managing from the master site, not a slave site. I had everything functioning well and I was testing under joomla 1.7. I did a clean rebuild of my master site based on Joomla 2.5.1 because I didn't want to deal with migration, I have not built any production sites, been waiting for everything to be stable on 2.5 so I didn't need to migrate later.

Primary question is, what could be causing this warning? and is it a critical stopper for me? is it an irrelevant warning or an indicator of a more critical issue?

I created a slave anyway and it seemed to work fine.

This is not critical.
I will try fix that in a next release to avoid this warning displayed.

I suspect that when there is no slave site at all, the computation of the combo box with the list of "owners" (top right of the manage site) would generate this warning.

I will try reproducing it.