The VanillaCommandWrapper#getListener(CommandSender) method will return null when the "sender" argument is a custom CommandSender. This will cause an NPE when calling Bukkit#dispatchCommand(CommandSender, String) with that CommandSender.

E.g:

public class CustomCommandSender implements CommandSender() {
...
}

CommandSender sender = new CustomCommandSender();

Bukkit.dispatchCommand(sender, "save-all");
// NPE at VanillaCommandWrapper#execute(CommandSender, String, String[]) (Line 42)