Only say a discussion was moved if it already exists

extensions/tags/src/Handlers/CategorySaver.php

@@ -26,7 +26,10 @@ class CategorySaver
             }
 
             $discussion->category_id = $categoryId;
+
+            if ($discussion->exists) {
                 $discussion->raise(new DiscussionWasMoved($discussion, $user, $oldCategoryId));
             }
         }
+    }
 }