Update getDefaultCurrencyForForm() to use standard form functions where available

CRM/Core/Payment/MJWTrait.php

@@ -196,25 +196,14 @@ trait CRM_Core_Payment_MJWTrait {
    *
    * @return string
    */
-  public function getDefaultCurrencyForForm($form) {
-    // For contribution pages it is in $form->_values
-    $currency = $form->_values['currency'] ?? NULL;
-    // If we AJAX load and have more than one processor (eg. stripe, pay later) we can end up with the wrong
-    // currency set in $form->_values. So we use $form->getVar('currency') to get the right one!
-    if (!$currency && is_a($form, 'CRM_Financial_Form_Payment')) {
-      $currency = $form->getVar('currency');
-    }
-
-    // Due to https://github.com/civicrm/civicrm-core/pull/21966 currency might be set to the string "undefined"
-    if (!$currency || $currency === 'undefined') {
-      // For event pages it is in $form->_values['event']
-      if (isset($form->_values['event'])) {
-        $currency = $form->_values['event']['currency'] ?? NULL;
-      }
+  public function getDefaultCurrencyForForm($form): string {
+    if (method_exists($form, 'getCurrency')) {
+      $currency = $form->getCurrency();
     }
-    if (!$currency || $currency === 'undefined') {
+
+    if (empty($currency) || $currency === 'undefined') {
       // If we can't find it we'll use the default from the configuration
-      $currency = CRM_Core_Config::singleton()->defaultCurrency;
+      $currency = Civi::settings()->get('defaultCurrency');
     }
     return $currency;
   }