v1.26.3

Fixes:
Fix crash on unknown MAD manifesto format (Launcher#749)
Fix typo in class member function (Launcher#750)
Check player validity in SetupChallenges_Threaded (Mods#819, Mods#820)
Define fs alias in source file where used (Launcher#747)

Contributors:
@Alystrasz @EladNLG @fvnkhead @GeckoEidechse @Jan200101 @Zanieon