Fix bad performance with TimerFileWatcher

TimerTaskTree.c

@@ -396,6 +396,7 @@ void timer_task_tree_set_data_path(TimerTaskTree *self, const char *path) {
     if (self->fileWatcher) {
         g_object_unref(self->fileWatcher);
     }
+    self->fileWatcher = NULL;
     self->fileWatcher = timer_file_watcher_new(path);
     g_signal_connect(self->fileWatcher, "file-changed", G_CALLBACK(data_file_updated), self);
 }