Problems when upgrading theme

Hello, I’m having issues when upgrading Portum from version 1.0.2 to whatever newer version.
I use a child theme that I created with Child Theme Configurator plugin

Analyzing both child and parent theme, the plugin gives me these four php alerts:

Notice: Undefined variable: header_class in /home/mhd-01/www.lieli.it/htdocs/wp-content/themes/portum/header.php on line 20

Notice: Undefined index: testimonials_column_stretch in /home/mhd-01/www.lieli.it/htdocs/wp-content/themes/portum/inc/helpers/class-portum-helper.php on line 176

Notice: Undefined index: testimonials_column_stretch in /home/mhd-01/www.lieli.it/htdocs/wp-content/themes/portum/inc/helpers/class-portum-helper.php on line 178

Notice: Undefined index: testimonials_column_stretch in /home/mhd-01/www.lieli.it/htdocs/wp-content/themes/portum/inc/helpers/class-portum-helper.php on line 180

The site seems to work well, until I upgrade Portum, then everything goes messed up (menus, graphics) and strange errors appears in every page.
I had to restore from backup two times after every upgrade attempt, any suggestions?

Hey there

these messages are just notifications, make sure debugger is disabled in wp-config.php