Hi @KathleenW ,
I have a test in my sample.
Expand the Table, you see It won't show error if value in [Notes] is not in html format.
If your code will give you error when values are not in html format. I suggest you to replace error with text error then create a new conditional column by if function.
Right click your PlainText column then use Replace Errors function.
Add Conditional column as below.
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.