DlgPrefInterface: Fix crash if no skin is available

[Holzhaus]

Only happens when neither the configured not the default skin is available (so that mixxx needs to shut down anyway), but should be fixed nonetheless.

[ronso0]

is just DlgPrefInterface not built or do we shut down with a warning?
indeed it makes no sense to start mixxx when skins are missing, not even to adjust other settings.

[Holzhaus]

As before the refactoring, SkinLoader calls reportCriticalErrorAndQuit() when no skins are found. However, this actually just queues shutdown. The construction of the whole MixxxApplication continues as normal. Only after everything is constructed and we're about to start the event loop, the "queue" is checked and we're actually shutting down.

So yes, we actually just abort the preferences construction, but it should work. I could also add pointer checks to the preferences implementation everywhere, but I doubt this would be worth it.

Instead, we should fix our skin loading logic in mixxx.cpp, because it we're detecting skins multiple times (first time for Mixxx itself, second time for the preferences). This leads to repeated error messages if the configured skin is missing. We should exit early if there is no valid skin missing, and the SkinLoader should remember the currently loaded skin. Then we can avoid executing the fallback logic in the preferences again, because we already know if the configured skin is working or not.

[daschuer]

That works good. LGTM
Thank you.