Use .at() instead of .value()

This is very minor improvement. about QList::value(qsizetype i): If you are certain that i is within bounds, you can use at() instead, which is slightly faster. about QStandardPaths::AppDataLocation: Returns a directory location where persistent application data can be stored. This is an application-specific directory. To obtain a path to store data to be shared with other applications, use QStandardPaths::GenericDataLocation. The returned path is never empty. On the Windows operating system, this returns the roaming path. This enum value was added in Qt 5.4.