Hi there @mayahtt,
Thanks for your patience so far.
This is not a problem we can replicate, which makes me wonder if perhaps there is a conflict with your theme or with another plugin. Here are a few things we can try.
Gather more information by temporarily enabling WP_DEBUG – then try to replicate the problem. My hope is that besides displaying the error you already cited, this will give us a ‘stack trace’ (basically, a bunch of additional information that might help us to track the source of the problem).
Perform some standard troubleshooting steps by temporarily deactivating all other plugins and switching to a standard, default theme such as Twenty Seventeen – then see if you can still replicate the error. If you cannot, it points to there being a conflict with one of the other components you have installed. Try reactivating them one by one and see if you can determine which is causing the conflict.
With regards to that last approach, if you prefer not to do this on a live site you could of course use a staging site, which can be as simple as a fresh WordPress installation contained within a subdirectory.
Let me know how you get on or if you have any questions about this approach!