To reproduce (on Oxygen and DD CI 8, at least):

Install Sirius plug-in (only the required plugin from the category is needed) and then the UML Designer plugin (just the UML designer itself, other plugins in the category are not needed). Install DD.

Create a new UML project and then a new class diagram. The palette panel in the editor is blank (there is nothing even selectable, so it is not just a color problem).

The palette can be recovered by leaving the editor open and then restarting eclipse or by maximizing the diagram editor (restoring the editor retains the palette, so this is the best workaround).