Add null check to MainNotifyIcon

fixes dlech/Keebuntu#16

I have an addon that completely disables the Winforms tray icon and replaces it with a native application indicator. The only way I could disable it was to make it null (and I tried many other ways first). So, when users use both addons, KeePassHTTP crashes because of this.

So hopefully you will accept this change so that both addons can live happily together.