Avoid magic numbers in ComponentSlotsForm

src/Element/ComponentSlotsForm.php

@@ -234,7 +234,11 @@ class ComponentSlotsForm extends ComponentFormBase {
    */
   public static function refreshAfterSourceAddition(array $form, FormStateInterface $form_state) {
     $parents = $form_state->getTriggeringElement()['#array_parents'];
-    $form = NestedArray::getValue($form, array_slice($parents, 0, -2));
+    $form_layers = count([
+      "(source)",
+      "add_more_button",
+    ]);
+    $form = NestedArray::getValue($form, array_slice($parents, 0, -$form_layers));
     return $form['sources'];
   }
 
@@ -257,7 +261,13 @@ class ComponentSlotsForm extends ComponentFormBase {
    */
   final public static function refreshAfterSourceRemoval(array $form, FormStateInterface $form_state) {
     $parents = $form_state->getTriggeringElement()['#array_parents'];
-    return NestedArray::getValue($form, array_slice($parents, 0, -4));
+    $form_layers = count([
+      "sources",
+      "(delta)",
+      "_remove",
+      "dropdown_actions",
+    ]);
+    return NestedArray::getValue($form, array_slice($parents, 0, -$form_layers));
   }
 
   /**