Update
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I updated the addon. The new version should not have the same problem. I'll try to add more advanced feature in next releases to make sure that storage can't be deleted by such an error.
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I'm really really sorry about the issue. I think I found a problem related to why history session might disappear suddenly and I will try to have a fix today, or later tomorrow. I really appreciate that you didn't go straight to 1 star and I promise that I will do my best to fix it as soon as possible. Will update this comment once the fix is ready.
Sadly I discovered after I launched the addon that addon storage seems not to be that reliable, having in mind that even by reinstalling the addon you can lose everything. I think I will add a form of secondary backup (or even some form of manual backup) to make sure things are more reliable.
I know many people rely on the auto-save function but the addon is mainly developed for manual saving of sessions as bookmark. I kinda made the wrong decision to provide an auto-saving feature at some point and sadly that created a lot more problem than I expected.
Again, really sorry, I will do my best to make sure things like this can't happen again.