When using the [gravityform] shortcode to embed a form in a post, and use a non existant id (perhaps the form was deleted after the post was created), then the gform_pre_render hook is fired with a null $form as an argument.
If this hook is employed by any plugins such as the WMPL Gravity forms multilingual or in our case a custom plugin then we have to check for a valid $form object before using it otherwise we get some nice exceptions thrown by php.
I dont know if this is by design, so I am reporting it as a probable bug -perhaps the hook should not be fired at all if the form is not found.
This occurs in the latest version of Gravity Forms. 1.6.11