Issue #3254971 by mrweiner, finne, John Pitcairn: PluginSelectWidget not allowing '- None- '.

src/Plugin/Field/FieldWidget/PluginSelectWidget.php

@@ -132,7 +132,7 @@ class PluginSelectWidget extends WidgetBase implements ContainerFactoryPluginInt
       ];
 
       // Only build the configuration form once a plugin ID has been selected.
-      if ($target_plugin_id !== NULL) {
+      if (!empty($target_plugin_id)) {
         $inline_form = $this->inlineFormManager->createInstance('plugin_configuration', [
           'plugin_type' => $plugin_type,
           'plugin_id' => $target_plugin_id,