[5.3] Fix check for JRouter class alias to work with b/c plugin switched off (joomla#44994)

Author: richard67

libraries/src/Router/Router.php

@@ -121,15 +121,13 @@ public static function getInstance($client, $options = [])
             // Create a Router object
             $classname = 'JRouter' . ucfirst($client);
 
-            if (!class_exists($classname)) {
-                throw new \RuntimeException(Text::sprintf('JLIB_APPLICATION_ERROR_ROUTER_LOAD', $client), 500);
-            }
-
-            // Check for a possible service from the container otherwise manually instantiate the class
+            // Check for a possible service from the container, otherwise manually instantiate the class if it exists
             if (Factory::getContainer()->has($classname)) {
                 self::$instances[$client] = Factory::getContainer()->get($classname);
-            } else {
+            } elseif (class_exists($classname)) {
                 self::$instances[$client] = new $classname();
+            } else {
+                throw new \RuntimeException(Text::sprintf('JLIB_APPLICATION_ERROR_ROUTER_LOAD', $client), 500);
             }
         }