Adding Drop-down text breaks my builds

[WriterAndrew]

I have an HTML Top-Nav output.
I'm experimenting with adding some Drop-down or Expanding text.

However I add the dropdown/expanding (either by creating an empty <p> tag and then clicking Insert > Drop-Down Text, or by creating a heading and some text and then clicking Drop-Down Text) when I build, it fails with the "Build failed: Object reference not sent to an instance of an object" error.

If I open the build log, it says there is an error in the target file (whichever Target I used for the build).

(If I then remove the added Drop-down text, I can again build as expected.)

Any suggestions on what might be causing this?
Thanks