TypeError: Cannot read property '$destroy' of null

First of all, thanks for this amazing plugin.

On:

```
// We can now clean up the scope
modalScope.$destroy();
```

Got:

```
TypeError: Cannot read property '$destroy' of null
at angular-modal-service.js:144
at processQueue (angular.js:16383)
at angular.js:16399
at Scope.$eval (angular.js:17682)
at Scope.$digest (angular.js:17495)
at angular.js:17721
at completeOutstandingRequest (angular.js:5964)
at angular.js:6243
```

Bye!

Start at angular-modal-service.js:144 and trace the modal cleanup path around modalScope.$destroy(). Reproduce the reported close flow, then verify that cleanup no longer attempts to call $destroy on a null scope and that the existing modal behavior remains intact.

Written by the indexing model from the issue text.