fix https://github.com/sagemathinc/smc/issues/1708 -- "sagews: exception in constructor of SynchronizedWorksheetCell"
fix https://github.com/sagemathinc/smc/issues/1708 -- "sagews: exception in constructor of SynchronizedWorksheetCell"

tscholl2/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc