diff -u b/panels_ipe/src/TempStoreTrait.php b/panels_ipe/src/TempStoreTrait.php
--- b/panels_ipe/src/TempStoreTrait.php
+++ b/panels_ipe/src/TempStoreTrait.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\panels_ipe;
 
+use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\panels\Plugin\DisplayVariant\PanelsDisplayVariant;
 
 /**
@@ -10,13 +11,6 @@
 trait TempStoreTrait {
 
   /**
-   * The module handler.
-   *
-   * @var \Drupal\Core\Extension\ModuleHandlerInterface
-   */
-  protected $moduleHandler;
-
-  /**
    * Returns the tempstore ID of a Panels IPE display.
    *
    * Modules will be allowed to alter the ID via
@@ -34,9 +28,17 @@
   protected function getTempStoreId(PanelsDisplayVariant $panels_display) {
     $id = $panels_display->id();
 
-    // Allow modules to determine the tempstore ID.
-    $module_handler = $this->moduleHandler ?: \Drupal::moduleHandler();
+    // Get the module handler. We cannot define a $moduleHandler property in
+    // this trait, or we could get fatal errors if classes that use this trait
+    // have a $moduleHandler property of their own.
+    if (isset($this->moduleHandler) && $this->moduleHandler instanceof ModuleHandlerInterface) {
+      $module_handler = $this->moduleHandler;
+    }
+    else {
+      $module_handler = \Drupal::moduleHandler();
+    }
 
+    // Allow modules to alter the tempstore ID.
     $module_handler
       ->alter('panels_ipe_tempstore_id', $id, $panels_display);