Unable to set as default theme

I am checking this issue.

Hi @coaston
I have added this patch for the fixes of your mentioned issue. Kindly Please Check.

Hi, I have resolved this issue and pushed the code to the issue branch.

The issue was in the isEmpty() as well as in theme_path functions in the file. Both the functions were no longer in use and the they have to be replaced.
I have remove those functions and added the updated version of those functions. I have also resolved the compatibility issue for Drupal 9.

Please review and verify the pushed code.

Hello,

Now I am experiencing different issue :

The website encountered an unexpected error. Please try again later.

Error: Call to undefined function drupal_get_path() in zadmin_preprocess_page() (line 56 of themes\contrib\zadmin\zadmin.theme).
call_user_func_array('zadmin_preprocess_page', Array) (Line: 285)
Drupal\Core\Theme\ThemeManager->render('page', Array) (Line: 436)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 204)
Drupal\Core\Render\Renderer->render(Array) (Line: 474)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 90)
__TwigTemplate_f3ff4d39c7414eb102db100b622027cb->doDisplay(Array, Array) (Line: 394)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 367)
Twig\Template->display(Array) (Line: 379)
Twig\Template->render(Array, Array) (Line: 40)
Twig\TemplateWrapper->render(Array) (Line: 53)
twig_render_template('themes/contrib/zadmin/templates/layout/html.html.twig', Array) (Line: 372)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 436)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 204)
Drupal\Core\Render\Renderer->render(Array) (Line: 158)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 583)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 159)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.view') (Line: 171)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 2) (Line: 74)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 2, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 2, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 2, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 2, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 2, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 2, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 2, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 2) (Line: 166)
Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber->makeSubrequest(Object, '/system/404', 404) (Line: 122)
Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber->on404(Object) (Line: 109)
Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase->onException(Object, 'kernel.exception', Object)
call_user_func(Array, Object, 'kernel.exception', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.exception') (Line: 224)
Symfony\Component\HttpKernel\HttpKernel->handleThrowable(Object, Object, 1) (Line: 89)
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: 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)

Move to RTBC.