How do I use $form_state['storage']['my_key']?
The example in the documentation link seemed to cause confusion.  Simply set your values in an associative array and use set/get storage.

$vals = [
  'name' =&gt; 'bob',
  'age' =&gt; 50
];

// save the data.
$form_state-&gt;setStorage($vals);


// retrieve the data
$vals = $form_state-&gt;getStorage();
$str = $vals['name'] . ' is ' . $vals['age'] . ' years old.';


https://www.cyberdev.co/how-to-store-temp-form-state-data