I got a continuous stream of this error while trying to build & preview a View displaying Chart, using a JSON file as the source in Query settings (thanks to module views_json_source):
Deprecated function: strip_tags(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\charts_billboard\Plugin\chart\Library\Billboard->populateData() (line 336 of /var/www/html/web/modules/contrib/charts/modules/charts_billboard/src/Plugin/chart/Library/Billboard.php)
#0 /var/www/html/web/core/includes/bootstrap.inc(158): _drupal_error_handler_real(8192, 'strip_tags(): P...', '/var/www/html/w...', 336)
#1 [internal function]: _drupal_error_handler(8192, 'strip_tags(): P...', '/var/www/html/w...', 336)
#2 /var/www/html/web/modules/contrib/charts/modules/charts_billboard/src/Plugin/chart/Library/Billboard.php(336): strip_tags(NULL)
#3 /var/www/html/web/modules/contrib/charts/modules/charts_billboard/src/Plugin/chart/Library/Billboard.php(83): Drupal\charts_billboard\Plugin\chart\Library\Billboard->populateData(Array, Array)
#4 /var/www/html/web/modules/contrib/charts/src/Element/Chart.php(202): Drupal\charts_billboard\Plugin\chart\Library\Billboard->preRender(Array)
#5 [internal function]: Drupal\charts\Element\Chart->preRender(Array)
#6 /var/www/html/web/core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php(101): call_user_func_array(Array, Array)
#7 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(788): Drupal\Core\Render\Renderer->doTrustedCallback(Array, Array, 'Render #pre_ren...', 'exception', 'Drupal\\Core\\Ren...')
#8 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(374): Drupal\Core\Render\Renderer->doCallback('#pre_render', Array, Array)
#9 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender(Array, false)
#10 /var/www/html/web/core/lib/Drupal/Core/Template/TwigExtension.php(477): Drupal\Core\Render\Renderer->render(Array)
#11 /var/www/html/web/sites/default/files/php/twig/64527c701f69d_views-view.html.twig_IlKsZx3xegJ5js0GgyIEbOPIi/JE-JNMmoyjYN9eZDUWJRl8drpZtL1ylDkgvrf1eaR70.php(110): Drupal\Core\Template\TwigExtension->escapeFilter(Object(Drupal\Core\Template\TwigEnvironment), Array, 'html', NULL, true)
#12 /var/www/html/vendor/twig/twig/src/Template.php(394): __TwigTemplate_c4d99210e0fec10dc526601045822ae5->doDisplay(Array, Array)
#13 /var/www/html/vendor/twig/twig/src/Template.php(367): Twig\Template->displayWithErrorHandling(Array, Array)
#14 /var/www/html/vendor/twig/twig/src/Template.php(379): Twig\Template->display(Array)
#15 /var/www/html/vendor/twig/twig/src/TemplateWrapper.php(40): Twig\Template->render(Array, Array)
#16 /var/www/html/web/core/themes/engines/twig/twig.engine(53): Twig\TemplateWrapper->render(Array)
#17 /var/www/html/web/core/lib/Drupal/Core/Theme/ThemeManager.php(372): twig_render_template('core/themes/cla...', Array)
#18 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(433): Drupal\Core\Theme\ThemeManager->render('views_view', Array)
#19 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(446): Drupal\Core\Render\Renderer->doRender(Array)
#20 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender(Array, false)
#21 /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(238): Drupal\Core\Render\Renderer->render(Array, false)
#22 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(580): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
#23 /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(239): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#24 /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(128): Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))
#25 /var/www/html/web/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php(90): Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))
#26 [internal function]: Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object(Symfony\Component\HttpKernel\Event\ViewEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#27 /var/www/html/web/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(111): call_user_func(Array, Object(Symfony\Component\HttpKernel\Event\ViewEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#28 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(168): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object(Symfony\Component\HttpKernel\Event\ViewEvent), 'kernel.view')
#29 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(74): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#30 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#31 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#32 /var/www/html/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#33 /var/www/html/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#34 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#35 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#36 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#37 /var/www/html/web/core/lib/Drupal/Core/DrupalKernel.php(686): Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#38 /var/www/html/web/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#39 {main}
.