I have two monitors and I like to keep my trays on one and the sketchup modeling window in the other. Just recently I've noticed that when the tray is not docked the images that preview the materials, styles, components, etc. are all blank. When the tray is docked they show up fine. I have the latest version of Sketchup 2018 and to my knowledge haven't had any updates recently. This just started happening. The only changes I've made are recently installing the TextureRandomizer and Cleanup3 plugins.
Images showing both conditions: