diff -u b/core/lib/Drupal/Core/Entity/ContentEntityForm.php b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
--- b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
@@ -352,7 +352,10 @@
     // entity should be deeply serialized as part of the form object
     // serialization.
     foreach ($form_display->getComponents() as $component) {
-      if (($widget_definition = $this->widgetPluginManager->getDefinition($component['type'])) && !empty($widget_definition['entity_deep_serialization'])) {
+      // There might be components that don't have a type property, therefore
+      // we have to check that the component has a type. This could occur by
+      // adding components e.g. through hook_entity_extra_field_info().
+      if (isset($component['type']) && ($widget_definition = $this->widgetPluginManager->getDefinition($component['type'])) && !empty($widget_definition['entity_deep_serialization'])) {
         $this->entityDeepSerializationRequired = TRUE;
         break;
       }