Only set about_to_close_all flag is there exists at least on view in current active window.
Only set about_to_close_all flag is there exists at least on view in
current active window.

hoanhtien/TypeScript-Sublime-Plugin,Microsoft/TypeScript-Sublime-Plugin,Microsoft/TypeScript-Sublime-Plugin,zhengbli/TypeScript-Sublime-Plugin,kungfusheep/TypeScript-Sublime-Plugin,hoanhtien/TypeScript-Sublime-Plugin,kungfusheep/TypeScript-Sublime-Plugin,Microsoft/TypeScript-Sublime-Plugin,zhengbli/TypeScript-Sublime-Plugin,zhengbli/TypeScript-Sublime-Plugin,hoanhtien/TypeScript-Sublime-Plugin,kungfusheep/TypeScript-Sublime-Plugin