TreePlugin: Entries disappearing

When using TreePlugin, I realized that one subtopic 2 levels down isn't shown.

Investigating further, i found out that

• as soon as you save the "lost" topic, it shows under TreePlugin, but vanishes again on reload
• the topic in question had an umlaut in its topic name

Removing the umlaut quickfixed the problem. Still this might be a bug to be fixed.