I have noticed something new. When the "Edit column data externally as XML File?" prompt comes up, if I click "No" and I have the padlock unlocked, I am able to edit XML in PL/SQL Developer's Large Data Editor. I didn't realize that. I thought that the native editor was read-only and I needed to install another tool - like Notepad++ in order to edit XML.
My goal is just to be able to edit data. I don't care what editor I use. If I can do that within PL/SQL Developer's Large Data Editor then I don't need to configure Notepad++ at all.
When I query my data, it does make me select the rowid from the table in order to edit and save XML, which is kind of a pain, but I can live with that.