To fix this warning message originated from the computed_field module: Deprecated function: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\Component\Utility\Html::escape() (line 424 of core/lib/Drupal/Component/Utility/Html.php).

computed_field-fix-deprecation-warnings.patch

diff --git a/src/Plugin/Field/FieldFormatter/ComputedStringFormatter.php b/src/Plugin/Field/FieldFormatter/ComputedStringFormatter.php
index 36be5e9..697c3ba 100644
--- a/src/Plugin/Field/FieldFormatter/ComputedStringFormatter.php
+++ b/src/Plugin/Field/FieldFormatter/ComputedStringFormatter.php
@@ -57,9 +57,9 @@ class ComputedStringFormatter extends ComputedFormatterBase {
   protected function prepareValue($value) {
 
     if ($this->getSetting('sanitized')) {
-      return nl2br(Html::escape($value));
+      return !empty($value) ? nl2br(Html::escape($value)) : $value;
     } else {
-      return nl2br($value);
+      return !empty($value) ? nl2br($value) : $value;
     }
   }