Old settings might cause new extension versions to crash

[ifl0w]

It seems that previously stored settings can cause issues with changes introduced in #143

More context can be found in this discussion: #143 (comment)

Should check what can be done to avoid such issues. Maybe we could reset stored settings or just use a schema id so that the old settings would not influence the extension. Obviously, this would also clear the previous history for all users. Maybe there is a better way to work around this issue.

[Lucki]

I looked briefly into it:
I think that happened because I've renamed the gsetting, and it was returning by default an empty path.
The jsonpath then returns directly because of the empty string with a null and then the genericJson is missing the crucial null check. The null is now checked because TypeScript mocked me there.
The schemaId actually will change later on too because they need a dynamic path to support multiple different source settings for the same source type. We can't (and shouldn't?) carry them over easily.
The history is still the same and should still work as before. I have to recheck which elements I've added to the history object.

Edit: I did add a name for the history entry which is used for blocking and saving images. That already was checked against null and is now properly typed for null. The history should be fine. Source settings will be "lost"* however.

*They're still in the dconf but not accessible through the UI.