"Undo" button does not work after clicking "Empty"

[pradeeban]

To reproduce:

• Create a study/graph.
• Empty the workspace using "Empty" option.
• Try to undo the "Empty" option by clicking the "Undo" button.

Expected behavior:

• The study/graph to reappear.

Current behavior:

• Nothing appears, and the workspace remains empty, despite the "Undo" button being enabled all the time.

[pradeeban]

Fixed by #154