Fix a null reference exception in UIConfig.cs

[MGGSK]

• UiConfig.Save no longer reloads the settings after saving. This change fixes a null reference exception if App.MainWindow is null (As pointed out by this comment: FIXME: For some reason unknown to me, App.MainWindow is null here, but only if the Settings were generated before e.g. on first launch above the line where the loading is done). Not using the window in UiConfig.Save fixes this issue.
• The settings are now only reloaded in UiConfig.Set