ev-sidebar-thumbnails: fix criticals in tree view mode
requested to merge 1042-criticals-printed-on-console-for-each-thumbnail-when-in-tree-view-mode into master
Lots of criticals printed from trying to use icon_view widget when in tree view mode.
This was a slip over from commit 6480c703
Fixes #1042 (closed)
Closes #1042 (closed)