declare 
  object_exists exception;
  except_01451  exception;

  pragma exception_init (object_exists , -01430);
  pragma exception_init (except_01451  , -01451);
begin 
  execute immediate 'ALTER TABLE AG_VENCIMENTOSDIV_CTAS_RECEBER MODIFY (AGENTECOBRADOR NULL)';

  exception 
    when object_exists  then null;
    when except_01451  then null;
end;