Variable: invalid state

It's possible for a Variable to enter an invalid state:

```
var v = new Variable();
// v.$ == '' (empty string)

var data = v.get()
// data == '' (empty string)

var v.set( data )
// throws
```

It should be possible to export a Variable and then import it again?

The same issue happens if `v.set()` throws as it leaves `v.$ == ''`
