Updated Allow.php

Typecast the $this->_getAllowedCurrencies() to an array so that single string responses don't break the logical checks

Author: deriknel

app/code/Magento/Config/Model/Config/Backend/Currency/Allow.php

@@ -51,7 +51,7 @@ public function __construct(
     public function afterSave()
     {
         $exceptions = [];
-        foreach ($this->_getAllowedCurrencies() as $currencyCode) {
+        foreach ((array)$this->_getAllowedCurrencies() as $currencyCode) {
             if (!in_array($currencyCode, $this->_getInstalledCurrencies())) {
                 $exceptions[] = __(
                     'Selected allowed currency "%1" is not available in installed currencies.',
@@ -60,7 +60,7 @@ public function afterSave()
             }
         }
 
-        if (!in_array($this->_getCurrencyDefault(), $this->_getAllowedCurrencies())) {
+        if (!in_array($this->_getCurrencyDefault(), (array)$this->_getAllowedCurrencies())) {
             $exceptions[] = __(
                 'Default display currency "%1" is not available in allowed currencies.',
                 $this->_localeCurrency->getCurrency($this->_getCurrencyDefault())->getName()