fix: error when no 'customVars' was set

app/controller/page.php

 
         $folder->prepare_files();
 
-        foreach($f3->get('customVars') as $key=>$value) {
-            $f3->set($key,$value);
-            $this->register_key($key,$folder);
+        if ( null !== $f3->get('customVars')) {
+            foreach($f3->get('customVars') as $key=>$value) {
+                $f3->set($key,$value);
+                $this->register_key($key,$folder);
+            }
         }
-        
+
         $folder->fill_content();
 
         foreach($f3->get('siteColors') as $k=>$v){