Fixing error when get_class is passed a string

src/Plugin/WebformElement/BootstrapDateTime.php

@@ -173,7 +173,7 @@ class BootstrapDateTime extends DateBase {
       $value = ($value['object']) ? $value['object']->format(DateFormat::load('html_datetime')->getPattern()) : '';
     }
     elseif ($value) {
-      if (get_class($value) == 'Drupal\Core\Datetime\DrupalDateTime') {
+      if (is_object($value) && get_class($value) == 'Drupal\Core\Datetime\DrupalDateTime') {
         $value = $value->format(DateFormat::load('html_datetime')->getPattern());
       }
       else {