If select NONE theme, will get error.

[WingLim]

Problems summary

In bin/generator, there is a problem.
User can select no theme, then will return an error like this:

panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
main.GetColorScheme(0x6f6e80, 0x0, 0x0, 0x1a4, 0xc00018c108)
        /Users/stephen/.config/nvim/cmd/main.go:134 +0x422
main.colorscheme(0xc0000b4060, 0x26)
        /Users/stephen/.config/nvim/cmd/main.go:122 +0x4a1
main.main()
        /Users/stephen/.config/nvim/cmd/main.go:21 +0x8b

Maybe you should add a default option in survey.MultiSelect or add some code to handle it if user selcet nothing.

Environment Information

• OS: Ubuntu 20.04

• thinkvim :echo g:thinkvim_version output: 2.6.5

• neovim :version output: 0.4.3

• :checkhealth or :CheckHealth result(neovim only):

How to reproduce the problem from neovim startup (Required!)

1. foo
2. bar
3. baz

Screenshot (if possible)