NEW HasOneRelationFieldInterface

src/Forms/HasOneRelationFieldInterface.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace SilverStripe\Forms;
+
+/**
+ * Added to form fields whose values are the ID of a has_one relation
+ * This is used in RequiredFields validation to check if the value is set
+ */
+interface HasOneRelationFieldInterface
+{
+}

src/Forms/RequiredFields.php

@@ -2,6 +2,7 @@
 
 namespace SilverStripe\Forms;
 
+use SilverStripe\LinkField\Form\LinkField;
 use SilverStripe\ORM\ArrayLib;
 
 /**
@@ -117,7 +118,7 @@ class RequiredFields extends Validator
                 }
             } else {
                 $stringValue = (string) $value;
-                if ($formField instanceof TreeDropdownField) {
+                if (is_a($formField, HasOneRelationFieldInterface::class)) {
                     // test for blank string as well as '0' because older versions of silverstripe/admin FormBuilder
                     // forms created using redux-form would have a value of null for unsaved records
                     // the null value will have been converted to '' by the time it gets to this point

src/Forms/TreeDropdownField.php

@@ -55,7 +55,7 @@ use SilverStripe\ORM\Hierarchy\MarkedSet;
  * @see CheckboxSetField for multiple selections through checkboxes.
  * @see OptionsetField for single selections via radiobuttons.
  */
-class TreeDropdownField extends FormField
+class TreeDropdownField extends FormField implements HasOneRelationFieldInterface
 {
     protected $schemaDataType = self::SCHEMA_DATA_TYPE_SINGLESELECT;