diff --git a/.cspell-project-words.txt b/.cspell-project-words.txt
index 767574007085da2d4e62c587b49150fc03be4f09..d845d4979ccef6ad71d6b8ff316510baab438501 100644
--- a/.cspell-project-words.txt
+++ b/.cspell-project-words.txt
@@ -9,4 +9,5 @@ penwidth
 pseudofield
 pseudofields
 rankdir
+safeid
 xurizaemon
diff --git a/config/schema/migrate_visualize.schema.yml b/config/schema/migrate_visualize.schema.yml
index b451cc489f9114fc0a63d25c4762ffed3c83c479..db5a2e5d5f28c0cf817aa3871ad68ed9b2a3925b 100644
--- a/config/schema/migrate_visualize.schema.yml
+++ b/config/schema/migrate_visualize.schema.yml
@@ -4,4 +4,5 @@ migrate_visualize.settings:
   mapping:
     display_mode:
       type: text
-      label: 'The output mode to use'
+      label: 'Display mode'
+      langcode: en
diff --git a/src/MigrateGraph.php b/src/MigrateGraph.php
index c7b4d4d4e8055f3a4c9cd75476807e415b5cf7ad..277dc923600ea705894cf8c5ca9b46dcb0565216 100644
--- a/src/MigrateGraph.php
+++ b/src/MigrateGraph.php
@@ -84,11 +84,13 @@ class MigrateGraph {
   /**
    * Make graph IDs "safe" (for MermaidJS).
    *
-   * @param $id
+   * @param string $id
+   *   An ID of a graph item.
    *
-   * @return array|string|string[]|null
+   * @return string
+   *   An ID, safe for use with MermaidJS.
    */
-  public function safeId($id) {
+  public function safeId(string $id) {
     return preg_replace('/[^a-zA-Z0-9_]/', '_', $id);
   }
 
@@ -299,7 +301,7 @@ class MigrateGraph {
    * @param \Fhaculty\Graph\Vertex $additionalSourceVertex
    *   (Optional) An additional source vertex. Used for chained sub-processes.
    */
-  protected function graphProcessPlugin(string $destinationPropertyName, MigrateProcessInterface $plugin, Vertex $destinationVertex, int $pipelineStepId, array $plugins, Vertex $additionalSourceVertex = NULL) {
+  protected function graphProcessPlugin(string $destinationPropertyName, MigrateProcessInterface $plugin, Vertex $destinationVertex, int $pipelineStepId, array $plugins, ?Vertex $additionalSourceVertex = NULL) {
     $graph = $this->getGraph();
 
     /** @var ProcessPluginBase $processPlugin */