Steps to reproduce the issue

Enable Maximum error reporting.
Enable System - Language Code.
Enable System - Language Filter.
Install German language.
Install Blog Sample Data.
Edit a category that has language All.
Click Associations button.

Actual result

Deprecated: SimpleXMLElement::addAttribute(): Passing null to parameter #2 ($value) of type string is deprecated in \administrator\components\com_categories\src\Model\CategoryModel.php on line 483

Deprecated: SimpleXMLElement::addAttribute(): Passing null to parameter #2 ($value) of type string is deprecated in \administrator\components\com_categories\src\Model\CategoryModel.php on line 483

Deprecated: explode(): Passing null to parameter #2 ($string) of type string is deprecated in \administrator\components\com_categories\src\View\Category\HtmlView.php on line 149

System information (as much as possible)

From v4.3.4 to v5.0