Update __isset function to comply with the same checks as __get (#4514)

Credit to @RickAcb.

modules/cms/classes/CodeBase.php

@@ -149,12 +149,20 @@ class CodeBase extends Extendable implements ArrayAccess
     }
 
     /**
-     * This will check if a property isset on the CMS Page object.
-     * @param  string  $name
-     * @return void
+     * This will check if a property is set on the CMS Page object.
+     * @param string $name
+     * @return bool
      */
     public function __isset($name)
     {
-        return isset($this->page->{$name});
+        if (isset($this->page->components[$name]) || isset($this->layout->components[$name])) {
+            return true;
+        }
+
+        if (isset($this->page->{$name})) {
+            return true;
+        }
+
+        return array_key_exists($name, $this->controller->vars);
     }
 }