Warning: Creating default object from empty value in…

Hello all.
Im having a problem with this plugin and i dont know how to fix it.
When i slick on any created category, im getting following error:
Warning: Creating default object from empty value in C:\xampp\apps\wordpress\htdocs\wp-includes\post.php on line 1691

Warning: Creating default object from empty value in C:\xampp\apps\wordpress\htdocs\wp-includes\post.php on line 1691
Also, when i go to:http://127.0.0.1/wordpress/dwqa-questions/Title of the page is “Archives” and i cant seam to be able to change it.Bare in mind that im just a beginner and any technical answers will have to be pretty basic.I found that i might be able to fix this by changing the way error messages are displayed and to set them to null so that they will not show, but again, i dont know how to do this. Can i edit posts.php to do this or is there another way.Thanks in advance !!