I can't figured it out myself. I already updated slick.js through github. It was 1.5.7 and now it is version 1.9.0. But i think it's related to jquery. So i don't know what the consequences are when i update jquery.
The error when i do an inspect:
JQMIGRATE: Migrate is installed, version 1.4.1
slick.js:1331 Uncaught TypeError: Cannot read property 'add' of null
at Slick.initADA (slick.js:1331)
at Slick.init (slick.js:1312)
at new Slick (slick.js:181)
at a.fn.init.$.fn.slick (slick.js:3027)
at HTMLDocument.<anonymous> ((index):2348)
at i (jquery.min.js?010a140e4d1643ac53739f38ea0cea4d:2)
at Object.fireWith [as resolveWith] (jquery.min.js?010a140e4d1643ac53739f38ea0cea4d:2)
at Function.ready (jquery.min.js?010a140e4d1643ac53739f38ea0cea4d:2)
at HTMLDocument.K (jquery.min.js?010a140e4d1643ac53739f38ea0cea4d:2)
The problem or the above error only occurs when i set in the concerning module > tab advanced > display "slide", when i set for example the setting to articlelist, infinity or slideshow the error is not visible.