I had this error yestoday and I think it is caused by uncompatible plugins.
I fixed it through banning all of plugins and then active them one by one to find out which plugin made the error. Finally I uninstalled them and fixed.
Hope it is helpful.
Is there a list of currently supported themes somewhere? When I am searching I am only finding stuff that has not been updated in many years.
Even with the 4 themes that come with NodeBB, it seems like only Persona is updated regularly.
I wanted to use Lavender but it has not been updated in long time and the link for the guide does not work.