Thanks for trying the new Ninja RSS Syndicator and for your feedback.
This is not an error but just a notice/warning.
It just points to a undefined variable which doesn't to seem present.
I've tried to find out what could cause this message in the specific code of Ninja RSS.
As far as I could discover, this might have to do with the Feeds overview/listing on the Feeds overview page.
In there you will find the dropdown box at the bottom which says Display #.
Now, if you have just 1 or 2 feeds created then it's most likely there isn't a notice message displaying.
In case you have more than 5 feed items, try to set the dropdown to view "All" items and see if the notice will be gone.
In general you shouldn't worry about this message/notice because it will not affect the main functionality of Ninja RSS.
I hope this helps.
Let us know how it goes.