Custom form block not working
The php file of the form is in a subfolder, but the class name in getForm() does not reflect this.

Change the location of the php file from

custom_module/src/Form/UserType/UserType.php


to

custom_module/src/Form/UserType.php


and 

namespace Drupal\custom_module\Form\UserType;


to

namespace Drupal\custom_module\Form;