I am trying to pragmatically set the revision log message after values of certain fields are changed (thus if a user has edits certain fields). More information is in the code comments

 function home_hub1_form_alter(&$  form, FormStateInterface $  form_state, $  form_id){   //    Hide Revisions log message text box $  form['revision_log_message']['#access']= FALSE;  //set revision check box to false since we are creating a revision progeammatically $  form['new_revision'] = array(     '#type' => 'hidden',     '#value' => FALSE );  // monitor every field if its from edit if ($  form_id == 'device_entity_edit_form'){      $  form['actions']['submit']['#submit'][] = 'home_hub1_form_device_entity_edit_submit';    } }   function home_hub1_form_device_entity_edit_submit(array &$  form, FormStateInterface $  form_state){  $  form_state->setTemporaryValue('entity_validated', TRUE);  //check if a field has changed and if it has write a revision log message if ($  form['field_city']['widget']['0']['value']['#value'] != $  form['field_city']['widget']['0']['value']['#default_value']) {      $  message = 'CITY changed from: ' . $  form['field_city']['widget']['0']['value']['#default_value'] . ' to: ' . $  form['field_city']['widget']['0']['value']['#value'];     $  form_state->setValue('revision_log_message', $  message);   } 

I am getting the following error: Error: Cannot unset string offsets in Drupal\Core\Field\WidgetBase->extractFormValues()

I also tried to get the node object so that i create a revision directly from it like below:

function home_hub1_form_device_entity_edit_submit(array &$  form, FormStateInterface $  form_state){  $  node = $  form_state->getFormObject()->getEntity();  //check if a field has changed and if it has write a revision log message if ($  form['field_city']['widget']['0']['value']['#value'] != $  form['field_city']['widget']['0']['value']['#default_value']) {  $  message = 'CITY changed from: ' . $  form['field_city']['widget']['0']['value']['#default_value'] . ' to: ' . $  form['field_city']['widget']['0']['value']['#value'];    $  node->setRevisionUserId(\Drupal::currentUser()->id());   $  node->setRevisionCreationTime(REQUEST_TIME);   $  node->setNewRevision(TRUE); // enabling revision for the entity save.   $  node->setRevisionLogMessage($  message); 

}

How can I create a revision after form submission?