Okay, restore the file to its original state and then try commenting out the session start like so:
// session_start();
The only thing that this will affect is the "remembering" of form field values. By default when a visitor submits a post and there is an error, the field values will be restored or remembered after the page reloads. Commenting out the session start will disable that functionality, but also prevent the error and get things working with your theme.
In the meantime I'll find a better solution for the next plugin update.