You may want to just give the title pane its own id in the function:
_createTitlePane: function (title, position, open, optclass, paneId) {
var tp = new TitlePane({
title: title,
id: paneId, // TitlePane id here
open: open
}).placeAt(this.sidebar, position);
domClass.add(tp.domNode, 'titlePaneBottomFix titlePaneRightFix');
if (optclass) {
domClass.add(tp.domNode, optclass);
}
tp.startup();
return tp;
}
Then you can refer to it with and hide it with:
esri.hide(dijit.byId("theIdYouGaveIt").domNode);
To understand the difference between dojo.byId and dijit.byId, this link may help.