How to use the result of getAssignedActivities() to get the value of the ID field

hi All,

I used the getAssignedActivities() method from outside aras to find 14 pieces of data.
But when I try to get the ID field using getItemByIndex(i).getProperty("ID"), he returns null

//my code

Item activities = inn.getAssignedActivities("Active", "B7E018BB45C40BB6BC48E3CA2612A971");

int actCnt = activities.getItemCount();
if (actCnt > 0)
   for (int i = 0; i <= actCnt; i++)
      string actId = activities.getItemByIndex(i).getProperty("ID");