The last Askanier elector of Saxony (Albrecht III) died in 1422. He'd married his wife in 1420 and there were no children. His brother and predecessor, Rudolf III, had three sons (Rudolf Jr, Wenzel and Sigmund) but all predeceased him. Of course according to some some sources, Rudi Sr was also pissed because the Emperor wouldn't let him wed Anna of Bohemia (OTL wife of Richard II) while others say it was another duke of Saxony. But let's assume that either Rudolf leaves a surviving son, or isn't poisoned in 1419 and remarries to someone besides his OTL second wife, or Albrecht lives long enough that he and his wife have kids (I'm not sure what killed him OTL). Either way, the electorate of Saxony isn't up for grabs for Emperor Sigmund to redistribute to the Wettins.