Issue #3225381 by mikelutz, jrockowitz, yivanov, Spokje, larowlan, longwave,...

Issue #3225381 by mikelutz, jrockowitz, yivanov, Spokje, larowlan, longwave, parkh, AdamPS, gcb:  PHP Notice logged when switching "Configuration type" in single configuration export screen

(cherry picked from commit ddd29c4c)

core/modules/config/src/Form/ConfigSingleExportForm.php

@@ -136,7 +136,7 @@ public function buildForm(array $form, FormStateInterface $form_state, $config_t
    */
   public function updateConfigurationType($form, FormStateInterface $form_state) {
     $form['config_name']['#options'] = $this->findConfiguration($form_state->getValue('config_type'));
-    unset($form['export']['#value']);
+    $form['export']['#value'] = NULL;
     return $form;
   }