It was reported that pressing "Undo" / ctrl + z after undoing all changes from filling segmentations creates an error. (reported in the "Fill-Erase-Close" checklist).
I played around with this and also got an error, but can't reproduce it now.

The general approach of this is to:

• create a segmentation with various spaces to fill / erase (easiest way is to load Pic3D-labels_multilabel-multistate-multilayer.nrrd from the test data directory)
• use the "fill" tool to fill various parts of the segmentation
• use the "undo" functionality to revert these changes
• press "undo" or ctrl + z once more after all changes have been reverted
• -> an error happens sometimes, apparently

I can't get this to work reliably, though. It happened once, but now I can't get it again, and was reported from the checklist. Needs further investigation.