category post not showing proper page on "preview changes"

Hello dear, I;m Developing my own little theme,I'm struggling on this issue.. When I uploaded a post and keep that posts on a specific category. preview changes/view post take me to index.php rather than category.php?? I'm not understanding why this happening, While if I click on that category from menu, that category posts only shows right place ("category.php").

I'm currently running homepage as index.php.