Set poller to null after free

scenes/seader_scene_read_14a.c

@@ -56,6 +56,7 @@ void seader_scene_read_14a_on_exit(void* context) {
     if(seader->poller) {
         nfc_poller_stop(seader->poller);
         nfc_poller_free(seader->poller);
+        seader->poller = NULL;
     }
 
     // Clear view

scenes/seader_scene_read_mfc.c

@@ -60,6 +60,7 @@ void seader_scene_read_mfc_on_exit(void* context) {
     if(seader->poller) {
         nfc_poller_stop(seader->poller);
         nfc_poller_free(seader->poller);
+        seader->poller = NULL;
     }
 
     // Clear view

scenes/seader_scene_read_picopass.c

@@ -55,6 +55,7 @@ void seader_scene_read_picopass_on_exit(void* context) {
     if(seader->picopass_poller) {
         picopass_poller_stop(seader->picopass_poller);
         picopass_poller_free(seader->picopass_poller);
+        seader->picopass_poller = NULL;
     }
 
     // Clear view