Loading deleted object causes critical internal error

To reproduce:

``` php
$t = new midgard_topic();
$t->create();
$t->delete();
$t = new midgard_topic($t->id);
echo midgard_connection::get_instance()->get_error_string();
```

Expected result:

```
"Object does not exist."
```

Actual result:

```
Fatal error: Uncaught exception 'midgard_error_exception' with message 'Critical internal error.Object doesn't exist'
```
