I got the same issue and found that the plugin user-activity-log was at the origin of the problem.
As I reported here,the problem is related to the hooks:
- ual_shook_created_term
- ual_shook_edited_term
- ual_shook_delete_term
They return nothing instead of the given term when the taxonomy is ‘nav_menu’.
If you don’t use this plugin, check the others plugins that call for example the pre_insert_term filter. They should return something.
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