Issue #3233420: CSSClass option plugin fails to build if there are no options defined.

src/Plugin/StyleOption/CssClass.php

@@ -66,20 +66,33 @@ class CssClass extends StyleOptionPluginBase {
    */
   public function build(array $build) {
     $value = $this->getValue('css_class') ?? NULL;
-    $option_definition = $this->getConfiguration()['options'];
-    if (is_array($value)) {
-      $class = implode(' ',
-        array_map(function ($index) use ($option_definition) {
-          return $option_definition[$index]['class'] ?? NULL;
-        }, $value)
-      );
+    $available_options = $this->getConfiguration('options');
+    $classes = [];
+
+    // Handle open text field values.
+    if (!$available_options) {
+      $classes = $value;
     }
     else {
-      $class = $this->getConfiguration()['options'][$value]['class'] ?? NULL;
+      if (is_array($value)) {
+        $classes = array_map(function ($index) use ($available_options) {
+          return $available_options[$index]['class'] ?? NULL;
+        }, $value);
+      }
+      else {
+        $classes = $available_options[$value]['class'] ?? NULL;
+      }
+
     }
-    if (!empty($class)) {
-      $build['#attributes']['class'][] = $class;
+
+    if (!empty($classes)) {
+      // Ensure $classes is an array so it can be easily manipulated later.
+      $classes = is_array($classes) ? $classes : explode(' ', $classes);
+      foreach ($classes as $class) {
+        $build['#attributes']['class'][] = $class;
+      }
     }
+
     return $build;
   }