(fix): prevent error log spam regarding null being passed to dirname because of a missing variable

Author: rmpel

src/PrefillGravityForms/GravityForms/GravityFormsAddon.php

@@ -38,6 +38,13 @@ class GravityFormsAddon extends GFAddOn
      */
     private static $_instance = null;
 
+    /**
+     * The full path to the Add-On file.
+     *
+     * @var string
+     */
+    protected $_full_path = __FILE__;
+
     /**
      * Singleton loader.
      */