Add method to indicate completeness.

app/Import/Prerequisites/FilePrerequisites.php

@@ -75,6 +75,17 @@ class FilePrerequisites implements PrerequisitesInterface
         return false;
     }
 
+    /**
+     * Indicate if all prerequisites have been met.
+     *
+     * @return bool
+     */
+    public function isComplete(): bool
+    {
+        // has no prerequisites, so always return true.
+        return true;
+    }
+
     /**
      * Set the user for this Prerequisites-routine. Class is expected to implement and save this.
      *