diff --git a/src/Form/AddTermsToVocabularyForm.php b/src/Form/AddTermsToVocabularyForm.php
index 1583d7e40a50fa91e80b2f2c9dcd4f5d85394f4a..c3a842d5ac7179249f66091050e02f94f27e9240 100644
--- a/src/Form/AddTermsToVocabularyForm.php
+++ b/src/Form/AddTermsToVocabularyForm.php
@@ -49,7 +49,7 @@ class AddTermsToVocabularyForm extends FormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, VocabularyInterface $taxonomy_vocabulary = NULL, $parents = []) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?VocabularyInterface $taxonomy_vocabulary = NULL, $parents = []) {
     // Cache form state so that we keep the parents in the modal dialog.
     // For non modals (non POST request), form state caching on is not allowed.
     // @see FormState::setCached()
diff --git a/src/Form/DeleteTermsForm.php b/src/Form/DeleteTermsForm.php
index 085c1d774858034bd61b13a905b9810cfc8f7c2f..a4544e883ecbd5d291169aaa5e820546955948dd 100644
--- a/src/Form/DeleteTermsForm.php
+++ b/src/Form/DeleteTermsForm.php
@@ -54,7 +54,7 @@ class DeleteTermsForm extends FormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, VocabularyInterface $taxonomy_vocabulary = NULL, $selected_terms = []) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?VocabularyInterface $taxonomy_vocabulary = NULL, $selected_terms = []) {
     if (empty($selected_terms)) {
       $form['info'] = [
         '#markup' => $this->t('Please select the terms you want to delete.'),
diff --git a/src/Form/ExportTermsMiniForm.php b/src/Form/ExportTermsMiniForm.php
index 03b6720b4483d7fb3f2ab80342255bcedf884618..0f16250dfd93dcdbde85c776c7bb46724a7b7241 100644
--- a/src/Form/ExportTermsMiniForm.php
+++ b/src/Form/ExportTermsMiniForm.php
@@ -42,7 +42,7 @@ class ExportTermsMiniForm extends FormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, VocabularyInterface $taxonomy_vocabulary = NULL, $selected_terms = []) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?VocabularyInterface $taxonomy_vocabulary = NULL, $selected_terms = []) {
 
     $form['voc'] = [
       '#type' => 'value',
diff --git a/src/Form/MoveTermsForm.php b/src/Form/MoveTermsForm.php
index 57bcdc041ec085d90064bb406f9a2a6e2b47d544..a5aaa770fa8fc39d024c201634f3387fdb56c56f 100644
--- a/src/Form/MoveTermsForm.php
+++ b/src/Form/MoveTermsForm.php
@@ -44,7 +44,7 @@ class MoveTermsForm extends FormBase {
    *
    * @todo Add autocomplete to select/add parent term.
    */
-  public function buildForm(array $form, FormStateInterface $form_state, VocabularyInterface $taxonomy_vocabulary = NULL, $selected_terms = []) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?VocabularyInterface $taxonomy_vocabulary = NULL, $selected_terms = []) {
     if (empty($selected_terms)) {
       $form['info'] = [
         '#markup' => $this->t('Please select the terms you want to move.'),
diff --git a/src/Form/TaxonomyManagerAdmin.php b/src/Form/TaxonomyManagerAdmin.php
index 0d1d413d48775a53a14328e406451de1495a6a07..798a151fd33179d784cc428b5443355c89af1bde 100644
--- a/src/Form/TaxonomyManagerAdmin.php
+++ b/src/Form/TaxonomyManagerAdmin.php
@@ -2,10 +2,10 @@
 
 namespace Drupal\taxonomy_manager\Form;
 
-use Drupal\content_translation\ContentTranslationManagerInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\content_translation\ContentTranslationManagerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
diff --git a/src/Form/TaxonomyManagerForm.php b/src/Form/TaxonomyManagerForm.php
index af15db64d52e07868fdcffc34a0fd05cd8758b32..344752f51777a9e71868c33e89d603f77b40e7c9 100644
--- a/src/Form/TaxonomyManagerForm.php
+++ b/src/Form/TaxonomyManagerForm.php
@@ -144,7 +144,7 @@ class TaxonomyManagerForm extends FormBase {
    * @return array
    *   The form structure.
    */
-  public function buildForm(array $form, FormStateInterface $form_state, VocabularyInterface $taxonomy_vocabulary = NULL) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?VocabularyInterface $taxonomy_vocabulary = NULL) {
     $current_user = $this->currentUser();
     $voc_list = [];
     $form['voc'] = [