diff --git a/src/Tests/NodeorderCrudTest.php b/src/Tests/NodeorderCrudTest.php
index 6343813..92a24da 100644
--- a/src/Tests/NodeorderCrudTest.php
+++ b/src/Tests/NodeorderCrudTest.php
@@ -20,7 +20,7 @@ class NodeorderCrudTest extends TaxonomyTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = ['taxonomy', 'node', 'nodeorder'];
+  protected static $modules = ['taxonomy', 'node', 'nodeorder'];
 
   /**
    * The node order manager.
diff --git a/tests/src/Functional/NodeorderPermissionsTest.php b/tests/src/Functional/NodeorderPermissionsTest.php
index bbc7f3d..8f0d76f 100644
--- a/tests/src/Functional/NodeorderPermissionsTest.php
+++ b/tests/src/Functional/NodeorderPermissionsTest.php
@@ -33,7 +33,7 @@ class NodeorderPermissionsTest extends BrowserTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = ['taxonomy', 'nodeorder'];
+  protected static $modules = ['taxonomy', 'nodeorder'];
 
   /**
    * {@inheritdoc}
diff --git a/tests/src/Kernel/NodeorderConfigSchemaTest.php b/tests/src/Kernel/NodeorderConfigSchemaTest.php
index fc2e672..1460ba8 100644
--- a/tests/src/Kernel/NodeorderConfigSchemaTest.php
+++ b/tests/src/Kernel/NodeorderConfigSchemaTest.php
@@ -17,7 +17,7 @@ class NodeorderConfigSchemaTest extends KernelTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = ['system', 'nodeorder'];
+  protected static $modules = ['system', 'nodeorder'];
 
   /**
    * The typed config handler.
@@ -52,7 +52,7 @@ class NodeorderConfigSchemaTest extends KernelTestBase {
     $config = $this->configuration->setData($data);
 
     $actual = TRUE;
-    $message = NULL;
+    $message = '';
     try {
       $this->assertConfigSchema($this->typedConfig, $config->getName(), $config->get());
     }
diff --git a/tests/src/Kernel/NodeorderInstallTestBase.php b/tests/src/Kernel/NodeorderInstallTestBase.php
index b4b10dc..6ff9d9c 100644
--- a/tests/src/Kernel/NodeorderInstallTestBase.php
+++ b/tests/src/Kernel/NodeorderInstallTestBase.php
@@ -14,7 +14,7 @@ abstract class NodeorderInstallTestBase extends KernelTestBase {
    *
    * @var array
    */
-  public static $modules = ['system', 'filter', 'text', 'taxonomy', 'user'];
+  protected static $modules = ['system', 'filter', 'text', 'taxonomy', 'user'];
 
   /**
    * The module installer.