fix activating tab when app was closed last time with "new tab"

src/public/javascripts/services/tree_cache.js

@@ -148,7 +148,7 @@ class TreeCache {
             else {
                 return this.notes[noteId];
             }
-        }).filter(note => note !== null);
+        }).filter(note => !!note);
     }
 
     /** @return {Promise<boolean>} */