How to get assignee in Problem Report task ?

Try this:

WTChangeActivity2 changeActivity = <get change activity object>;

Team team = (Team)changeActivity.getTeamId().getObject();

  ArrayList assignees = (ArrayList)team.getRolePrincipalMap().get(Role.toRole("ASSIGNEE"));

  for(Object u : assignees)

  {

       WTPrincipal user = (WTPrincipal)((WTPrincipalReference)u).getObject();

  }

You may need to change role names if you don't use ASSIGNEE as the role.