NULL initialize GError

NULL initialize GError before tying to parse to avoid a GLib warning, like this:

```
(process:21877): GLib-WARNING **: GError set over the top of a previous GError or uninitialized memory.                                              
This indicates a bug in someone's code. You must ensure an error is NULL before it's set.                                                                
The overwriting error message was: SQL code does not contain any statement                                                                                
*** RuntimeError Exception: error parsing sql

```