[Flow] Type mapping for ArrayCollection Objects

[Rocky]

Hi, 

The aggregate root object has many subObject. In the AggregateRoot.php I have below code:
    /**                                                                                                                
     * @var \Doctrine\Common\Collections\Collection<\My\App\Domain\Model\SubObject>                           
     * @ORM\OneToMany(mappedBy="aggregateRoot",cascade="all")                                                              
     */                                                                                                                
    protected $subObjects;  

The SubObject is an entity and has its own properties. In the New.html for AggregateRootController's new action, I have below code:
        <label for="subObjects.1.propertyA">Property A</label>                                                                      
        <f:form.textfield property="subObjects.1.propertyA" id="subObjects.1.propertyA" />                                               
        <label for="subObjects.1.propertyB">Property B</label>                                                                  
        <f:form.textfield name="subObjects.1.propertyB" id="subObjects.1.propertyB" />                                       
 
When I submit the form, I got below exception:
Uncaught Exception in Flow
#1297759968: Exception while property mapping for target type "My\App\Domain\Model\AggregateRoot", at property path "": Property "subObjects" having a value of type "Doctrine\Common\Collections\ArrayCollection" could not be set in target object of type "My\App\Domain\Model\AggregateRoot".

I have tried something like below code but no luck:
protected function initializeUpdateAction() {
        $commentConfiguration = $this->arguments['comment']->getPropertyMappingConfiguration();
       $commentConfiguration
        ->forProperty('items.*')
        ->setTypeConverterOption(
                'TYPO3\Flow\Property\TypeConverter\PersistentObjectConverter',
                \TYPO3\Flow\Property\TypeConverter\PersistentObjectConverter::CONFIGURATION_CREATION_ALLOWED,
                TRUE
        );
}

Any idea how can I make TYPO3 Flow do the type mapping for ArrayCollection objects, so I could create the whole aggregate with one form please?