Instantiating Session object in the constructor of ES doesn't work from version 8 and beyond. We had this issue when we did the upgrade from 2013 to 8.5. So what we did is moved it to all event subscriptions methods. Please use it like that and forget about using Core Service in event system because it is an abomination, a Frankenstein which shouldn't exist ...
Since, you are using DXA, I will say the approach of generating the sitemap might not makes sense (unless there are other fine prints which we are not aware about).
The correct approach could be similar to the one mentioned here:
Generate Sitemap XML File using default functions of DXA 1.5/1.7 in Web 8/8.5