fix: Initialize SchemaFormModel.formData

[ktaletsk]

References

Fixes #66

Code changes

User-facing changes

Backwards-incompatible changes

Chores

• [ ] linted
• [ ] tested
• [ ] checked on binder
• [ ] documented
• [ ] changelog entry

[bollwyvl]

Thanks!

I've included this in #69 (though it sets the private _formData member in the constructor to avoid creating events for an uninitialized instance)