Error when using events+

I get this error when using your plugin and try to preview a created event.

Notice: Undefined variable: theme in /home/oneversi/public_html/vedvarendeenergi/wp-content/themes/yootheme/header.php on line 10

Fatal error: Uncaught Error: Call to a member function get() on null in /home/oneversi/public_html/vedvarendeenergi/wp-content/themes/yootheme/header.php:10 Stack trace: #0 /home/oneversi/public_html/vedvarendeenergi/wp-includes/template.php(684): require_once() #1 /home/oneversi/public_html/vedvarendeenergi/wp-includes/template.php(643): load_template('/home/oneversi/...', true) #2 /home/oneversi/public_html/vedvarendeenergi/wp-includes/general-template.php(45): locate_template(Array, true) #3 /home/oneversi/public_html/vedvarendeenergi/wp-content/themes/yootheme/single.php(8): get_header() #4 /home/oneversi/public_html/vedvarendeenergi/wp-includes/template-loader.php(74): include('/home/oneversi/...') #5 /home/oneversi/public_html/vedvarendeenergi/wp-blog-header.php(19): require_once('/home/oneversi/...') #6 /home/oneversi/public_html/vedvarendeenergi/index.php(17): require('/home/oneversi/...') #7 {main} thrown in /home/oneversi/public_html/vedvarendeenergi/wp-content/themes/yootheme/header.php on line 10

I have tried disabling all plugins, with no success.

What can be wrong here?

best regards
Lars Thyregod