Opening the material selector after reloading files could cause long UI freezes, because some linked in materials don't have the preview stored in the source file. So Blender would keep rerendering it after every file load, which may involve compiling OpenGL shaders, which again freezes the UI typically. This was reported as quite an issue for the Heist Production by the Blender Studio. Don't render these missing material previews from linked data-blocks anymore. Differential Revision: https://developer.blender.org/D16538 Reviewed by: Brecht Van Lommel, Jeroen Bakker