Maxed out the tech tree, and after the final step, the game takes me to the tech tree screen.
There are, of course, no more "selectable" tech options. However, the large round button with a check through it, which closes the Tech Tree screen stays greyed out.
It is therefore impossible to return to the game, or do anything else! Serious bug.

Have you checked all the side-path techs? IIRC, you can progress to the third era without researching everything in the second (plus there are side paths in the third).