Static website https redirect and getting rid of html ending?

For some case i have installed SSL. But after opening webpage the secure symbol appear for 0,1 sec and then its unsecured connection again. Is it problem with redirecting in htaccess file?

Website link: avtobus-slovenija.si

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html [NC,L]
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]