Avoid error if item tree disappeared during refresh
Avoid error if item tree disappeared during refresh

rmzelle/zotero,hoehnp/zotero,gracile-fr/zotero,rmzelle/zotero,retorquere/zotero,hoehnp/zotero,retorquere/zotero,rmzelle/zotero,gracile-fr/zotero,retorquere/zotero,gracile-fr/zotero,hoehnp/zotero