Removed deprecated theme function

Problem/Motivation

Error when loading widget

User deprecated function: Theme functions are deprecated in drupal:8.0.0 and are removed from drupal:10.0.0. Use Twig templates instead of theme_checkbox_tree_label(). See https://www.drupal.org/node/1831138 in Drupal\Core\Theme\Registry->processExtension() (line 498 of core/lib/Drupal/Core/Theme/Registry.php).

Stack:

Drupal\Core\Theme\Registry->processExtension(Array, 'term_reference_tree', 'module', 'term_reference_tree', 'modules/contrib/term_reference_tree') (Line: 341)
Drupal\Core\Theme\Registry->build() (Line: 240)
Drupal\Core\Theme\Registry->get() (Line: 88)
Drupal\Core\Utility\ThemeRegistry->initializeRegistry() (Line: 69)
Drupal\Core\Utility\ThemeRegistry->__construct('theme_registry:runtime:b4mdb', Object, Object, Array, 1) (Line: 260)
Drupal\Core\Theme\Registry->getRuntime() (Line: 142)
Drupal\Core\Theme\ThemeManager->render(Array, Array) (Line: 431)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 200)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 226)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 227)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 117)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object) (Line: 78)
Symfony\Component\EventDispatcher\LegacyEventDispatcherProxy->dispatch(Object, 'kernel.view') (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
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: 57)
Drupal\advban\AdvbanMiddleware->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 705)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)