Fix bug where app would crash is Screen.new() was called with a non-hash argument

This fixes an instance where a user assigns an action to a table view cell and passes the arguments but doesn't specify a proper arguments hash:

```ruby
def table_data
[cells:[{
  title: open screen,
  action: :open_screen
}]]
end

def open_screen(args={})
  # args here is a UITableViewCell
  open MyScreen.new(args)
end
```