Assert\Expression gives Variable Not Valid around position 2 - forms

Question

I have a form. I am trying to assert that if one field is not null, then another field must also not be null. Trying to do this with Symfony annotations as I don't want the code in the controller and was told this would be a good way to do it.
I have tried using an Assert\Expression, however I keep getting various errors on the field that I am checking. It's slightly concerning that it says Variable when I need it to be referring to a field.
// The field that I want to check if it is null
'activeTestData',
null,
[
'label' => 'form.label.active_test_data',
'required' => false,
]
)
->add(
// The field that can't be null if the activeTestData field is not null
'activeTestDataUnit',
ChoiceType::class,
[
'label' => 'form.label.active_test_data_unit',
'required' => false,
'choices' => [
'form.label.active_test_please_select' => '',
'Byte(s)' => 'b',
'Kilobyte(s)' => 'k',
'Megabyte(s)' => 'm',
'Gigabyte(s)' => 'g',
],
// Where I am having the issue
'constraints' => [
new Assert\Expression([
'expression' => "!activeTestData == null",
'message' => 'Please enter a unit for active test data'
]),
],
I want a FORM error for when there is nothing in activeTestDataUnit but there is something in activeTestData. Unfortunately I get a symfony error:
Variable "activeTestData" is not valid around position 2 for expression !activeTestData == null.

Answer 1

Correct syntax for this situation would be the following:
"!this.getParent().get('activeTestData')->getData() == null"
You can see more examples and details on symfony documentation.
I updated my answer. In this context "this" is a Form object of a "activeTestDataUnit" field. So to check for a "activeTestData" value you have go to to the parent and then get a correct child.
As of Symfony 4.1 you can pass your custom values to the "values" argument and avoid all of this clutter, by passing $builder->getData(). See here for the more information on symfony documentation