UpdateService crashes when pre-release updates are disabled

[wackerl91]

Disabling pre-release updates on v0.6.0 leads to an immediate crash in the update service (and thus Luna not starting at all) because the API response isn't parsed properly.

Workaround: enabling pre-release updates in the settings (simply access them via the add-ons context menu) resolves this for now, essentially reverting to the behaviour on older versions.

A hotfix is underway and will be released soon(ish). Sorry for any inconvenience caused.