BUG: reset the default editor color node each time dialog is opened

This addresses an issue where the color combo box becomes unpopulated
when the scene is closed.

There is still an issue with the thumbnails of the colors are not visible
but this is an issue with the color combo box and not the editor.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23975 3bd1e089-480b-0410-8dfb-8563597acbee