How to hide the black line of the collapsed lines in Notepad++?

How do you remove a black horizontal line that always appears after you fold a block?

I have followed one of tips that said to use

Settings > Style Configurator > Global Styles > Default Style > Colour Style > Foreground Color: White`

but it made the filenames in Doc Switcher white too.

How do I make the black lines of folding disappear?

According to this thread,

1. Install LuaScript through Plugin Manager.
2. Open the startup script with Plugins > LuaScript > Edit Startup Script.
3. Add the two following lines,

editor1.FoldFlags = 0
editor2.FoldFlags = 0

I don't why it works, but it works.