How to fix TypeError: Cannot convert undefined or null to object

Hi,

For few weeks now I had some serious issues with uploading thumbnails and selecting them for posts. When I try to upload a thumbnail from a post editor, it doesn’t allow me, because the windows is blank.

When I try to load it from earlier uploaded media files it often crashes and gives my this error:

TypeError: Cannot convert undefined or null to object
    at g.cleanup (https://www.pixelophobia.pl/wp-includes/js/backbone.min.js?ver=1.4.0:1:3793)
    at g.off (https://www.pixelophobia.pl/wp-includes/js/backbone.min.js?ver=1.4.0:1:3651)
    at r (https://www.pixelophobia.pl/wp-includes/js/backbone.min.js?ver=1.4.0:1:2246)
    at l (https://www.pixelophobia.pl/wp-includes/js/backbone.min.js?ver=1.4.0:1:869)
    at h.Collection.n.off (https://www.pixelophobia.pl/wp-includes/js/backbone.min.js?ver=1.4.0:1:1636)
    at i.<computed> [as off] (https://www.pixelophobia.pl/wp-includes/js/media-views.min.js?ver=5.3.2:1:5236)
    at i.dispose (https://www.pixelophobia.pl/wp-includes/js/media-views.min.js?ver=5.3.2:1:23774)
    at i.dispose (https://www.pixelophobia.pl/wp-includes/js/media-views.min.js?ver=5.3.2:1:73988)
    at i.remove (https://www.pixelophobia.pl/wp-includes/js/media-views.min.js?ver=5.3.2:1:23830)
    at https://www.pixelophobia.pl/wp-includes/js/underscore.min.js?ver=1.8.3:1:2850

I know there have been mentions of TypeError, but in most cases it’s a technical explanation of what happened and there is no solution given. Could you help me out and let me know what to do?

The page I need help with: [log in to see the link]