Check that plugins are valid

Closes #3469151

src/Element/PluginConfig.php

@@ -47,7 +47,8 @@ class PluginConfig extends FormElement {
       ];
     }
 
-    $plugins = $form_state->getValue($element['#parents']) ? array_filter($form_state->getValue($element['#parents']), function($p) {return is_array($p) && isset($p['id']);}) : $element['#default_value'];
+    $plugins = $form_state->getValue($element['#parents']) ?: $element['#default_value'];
+    $plugins = array_filter($plugins, function($p) {return is_array($p) && isset($p['id']);});
     foreach ($plugins as $uuid => $plugin) {
       $element[$uuid] = [
         '#type' => 'details',