Prevent modification of snapshots when mounted as root subvolume

[0iSuck-at-coding0]

Is your feature request related to a problem? Please describe.
I booted into a snapshot using GRUB and I restored it using the Timeshift GUI. After I rebooted, I deleted the snapshot I restored into and my whole root subvolume got deleted. This resulted in an unbootable system. The problem was that the snapshot was still mounted as my root subvolume. Obviously, when deleted, GRUB had nothing to boot into.

Describe the solution you'd like
I would like the Timeshift GUI to stop you from modifying snapshots when a snapshot is mounted and in use.

Additional context
This GitHub post is by me and details my whole experience: Deleting snapshot causes loss of @ subvolume when restoring via GRUB