Cannot create Form with a Form Field. Occurs in dev env. with debug enabled.

Steps:
Log in as admin.
Go to Forms.
Create new form.
Fill Title.
Add "Single line input" form field
Publish form

Result:
Action results in an exception
```
ErrorException:
Notice: Undefined index: value

at vendor/ezsystems/ezplatform-form-builder/src/lib/FieldType/Storage/DoctrineGateway.php:208
at EzSystems\EzPlatformFormBuilder\FieldType\Storage\DoctrineGateway->insertValidator(5, array('identifier' => 'required'))
(vendor/ezsystems/ezplatform-form-builder/src/lib/FieldType/Storage/FormStorage.php:225)
at EzSystems\EzPlatformFormBuilder\FieldType\Storage\FormStorage->insertForm(60, 1, 265, 'eng-GB', array('fields' => array(array('identifier' => 'single_line', 'name' => 'Single line input', 'attributes' => array(array('identifier' => 'placeholder', 'value' => ''), array('identifier' => 'help', 'value' => ''), array('identifier' => 'default_value', 'value' => '')), 'validators' => array(array('identifier' => 'required'), array('identifier' => 'min_length'), array('identifier' => 'max_length'), array('identifier' => 'regex')), 'id' => 'fbf-4e592129-0c8a-bd89-1b00-04b3ff870dd0'))))
(vendor/ezsystems/ezplatform-form-builder/src/lib/FieldType/Storage/FormStorage.php:98)
```
(screen from Travis: https://res.cloudinary.com/ezplatformtravis/image/upload/v1561976414/screenshots/5d19de5e7cc9c401829836-vendor_ezsystems_ezplatform-form-builder_features_formbuildertests_feature_94_ggsege.png)