Add missing class option

[VincentLanglet]

Subject

When using the ModelHiddenType, the option class is required.

Changelog

### Fixed
- Correctly pass the class to the ModelHiddentType when using child admin without parentAssociationMapping.

[jordisala1991]

Can you add a failing tests? How did you encounter the error?

[VincentLanglet]

Can you add a failing tests? How did you encounter the error?

Honestly I dont really know fully how this is working.
A colleague was configuring a childAdmin but he was giving a wrong fieldName.
So he had an error message "option class is missing in ModelHiddenType".
By manually passing a class, he then got the message about the fact he were using a wrong fieldName, which was better to understand his mistake.

When the childAdmin is correctly configured, no error is thrown maybe because the option was set by something like
the AbstractFormContractor::getDefaultOptions.

I had thought if we should pass

'field_options' => [
     'model_manager' => $this->getModelManager(),
     'class' => $this->getClass(),
],

or

'field_options' => [
     'model_manager' => $this->getParent()->getModelManager(),
     'class' => $this->getParent()->getClass(),
],

But since we're trying to add a filter about the parent entity, I just tried and the second value was the right one.

Not sure about adding a test with a wrong config in order to expect we see an error message rather than another...

[jordisala1991]

I am asking because I use a lot the parent child thing (most of my admins are nested), and I never seen an error like this

[VincentLanglet]

I would say it's because you are using them correctly ^^

Try to modify a addChild call in your config to give a wrong fieldname ; it was the way for me to produce this error