Error installing new theme

Trying to install a new theme, I hit a certain setting and I get this message:

Warning:+Creating+default+object+from+empty+value+in+/home/myname/public_html/wp-content/plugins/revslider/inc_php/framework/update.class.php+on+line+35

Warning:+Cannot+modify+header+information+-+headers+already+sent+by+(output+started+at+/home/myname/public_html/wp-content/plugins/revslider/inc_php/framework/update.class.php:35)+in+/home/myname/public_html/wp-includes/pluggable.php+on+line+1121

I do not understand what I did wrong and thus can not straighten it.
I would appreciate some advice that would be helpful to clear the error.

With gratitude,