"Cannot add headers" error

I just bought the real estate theme, installed it, set it up, installed a child theme, added the slider option to the homepage, tried to create the 1st slide, when I go to insert from media library I see this error:
Warning: Cannot modify header information – headers already sent by (output started at /home/fob2438/public_html/***mydomain****/wp-admin/includes/template.php:1867) in /home/fob2438/public_html/***mydomain***/wp-content/themes/RT/framework/class/class_layout.php on line 139

I try to add an image and the window just remains blank. I can’t upload or insert from URL either as I get the same blank window.

Can you help please?