Problem/Motivation
The module throws this error on a site.
Deprecated function: Creation of dynamic property Drupal\do_username\DOComputedFields::$doUserService is deprecated in Drupal\do_username\DOComputedFields->__construct() (line 22 of modules/contrib/do_username/src/DOComputedFields.php).
Drupal\do_username\DOComputedFields->__construct(Object, 'bio', Object) (Line: 47)
Drupal\Core\TypedData\TypedData::createInstance(Object, 'bio', Object) (Line: 91)
Drupal\Core\TypedData\TypedDataManager->createInstance('string', Array) (Line: 100)
Drupal\Core\TypedData\TypedDataManager->create(Object, NULL, 'bio', Object) (Line: 200)
Drupal\Core\TypedData\TypedDataManager->getPropertyInstance(Object, 'bio') (Line: 66)
Drupal\Core\Field\FieldItemBase->__construct(Object, 'bio', Object) (Line: 47)
Drupal\Core\TypedData\TypedData::createInstance(Object, 0, Object) (Line: 91)
Drupal\Core\TypedData\TypedDataManager->createInstance('field_item:do_username', Array) (Line: 100)
Drupal\Core\TypedData\TypedDataManager->create(Object, NULL, 0, Object) (Line: 200)
Drupal\Core\TypedData\TypedDataManager->getPropertyInstance(Object, 0, Array) (Line: 82)
Drupal\Core\Field\FieldTypePluginManager->createFieldItem(Object, 0, Array) (Line: 41)
Drupal\Core\Field\FieldItemList->createItem(0, Array) (Line: 69)
Drupal\Core\TypedData\Plugin\DataType\ItemList->setValue(Array, ) (Line: 107)
Drupal\Core\Field\FieldItemList->setValue(Array, ) (Line: 208)
Drupal\Core\TypedData\TypedDataManager->getPropertyInstance(Object, 'field_do_username', Array) (Line: 74)
Drupal\Core\Field\FieldTypePluginManager->createFieldItemList(Object, 'field_do_username', Array) (Line: 600)
Drupal\Core\Entity\ContentEntityBase->getTranslatedField('field_do_username', 'x-default') (Line: 564)
Drupal\Core\Entity\ContentEntityBase->get('field_do_username') (Line: 181)
Drupal\Core\Entity\Entity\EntityFormDisplay->buildForm(Object, Array, Object) (Line: 121)
Drupal\Core\Entity\ContentEntityForm->form(Array, Object) (Line: 311)
Drupal\user\AccountForm->form(Array, Object) (Line: 107)
Drupal\Core\Entity\EntityForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm('user_form', Object) (Line: 283)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 592)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 182)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 53)
Asm89\Stack\Cors->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 704)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
Steps to reproduce
Just install the module and view a form where this field is present. This might also show up on other pages randomly (errors shown from session).
Proposed resolution
It's an easy enough fix. Define the property in the class as indicated by the error message.