Undo does not work after input-layer was deleted

[haesleinhuepf]

... which might be ok in general. We just need to make sure that a proper error message shows up. Currently, it tries to rebuild the workflow and fails. It shows an empty dialog instead.

To reproduce:

• Open napari + assistant
• Open blobs.tif example
• Do Noise Removal, Binarize, Label
• Delete all layers
• Click on "Undo"