Writer's naviagtor crashes when you click on a frame to select it,
because the SwContent* that is passed to SwContentTree::GotoContent()
is deleted because SidebarController::CreatePanels() re-creates
every single panel and then ResetPanels() disposes the existing ones,
which deletes the naviagtor's SwContent instances.

(regression from 536c259c)

Change-Id: I8f875df816a2d5948026a1d3f30b86bb2418acda