Fatal error: Call to a member function check_capabilities() on a non-object

When I try to edit my new theme (Sydney from aTheme) from my new web raich.cat my screen goes blanck but for this error message:

Fatal error: Call to a member function check_capabilities() on a non-object in /home/u450312825/public_html/wp-includes/class-wp-customize-control.php on line 283

I went to my files and serached for this class-wp-customize-control.php file.
Line 283 says:
if ( ! $setting->check_capabilities() )

But coding for me is like a foregin language, I just get the basics of it, so right now I’m pretty lost.

Is there anything I need to add to this file?
In case some more info is nedded I’ll copy from 281 to 285:
final public function check_capabilities() {
foreach ( $this->settings as $setting ) {
if ( ! $setting->check_capabilities() )
return false;
}