Using Version 6.0.9 (and the one before that), full version

OS: Windows 10 Pro 1903

Way of reproducing bug: Create another layer (layer 2). Do something in that layer. Hide layer 1. Undo previous action.

There are no error messages but you can no longer use undo or redo anything, breaking the functionality of the program until it restarts.
For some reason it renames the layer, making me think you have two undo vectors. But hey, I have no insight in your code.