Chek if a user has alrady created a node and redirect him to a page

Drupal8 : I try to redirect a user to a page if he has already created a node in mytheme.theme, in order to prevent a user to create a node twice :

function mytheme_node_prepare($  node) {   if ($  node->bundle() == "my_node") {     if (!isset($  node->nid)) {       $  existing_nid = db_query("SELECT nid FROM {node} WHERE (type = :type1) AND uid = :uid", array("type1" => "my_node", "uid" => $  node->uid))->fetchField();       if ($  existing_nid != FALSE) {         return new TrustedRedirectResponse('/mypage');       }     } 

this doesn’t work,

thanks for your help

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