Unable to select object
OK, then it's as I suspected. The "+" is some sort of graphical object or icon that is not something that is "mappable". You'll need to investigate further to find it and access it a different way.
Note that there is a property called "children" on that panel. That will include any child objects, even ones that are not mappable. I'd start there, looking for whether or not you can get to it.
Alternative solution: Keep in mind that your job as an automated tester is not necessarily to test whether or not a component the developers are using functions correctly... it's whether or not, once a user interacts with the component, that the softward does what it's supposed to do. So, is there another way of selecting the "+" functionality without clicking that button? A keystroke that would work instead? Attempt to find an alternative way around.