Adding IF(ISERROR) to multiple and non-sequential cell references

Is there an easy way to change

='Week 1'!$D$21
='Week 1'!$D$30
='Week 1'!$D$39
='Week 2'!$D$21
='Week 2'!$D$30
='Week 2'!$D$39

to

=IF(ISERROR('Week 1'!$D$21), "", ('Week 1'!$D$21))
=IF(ISERROR('Week 1'!$D$30), "", ('Week 1'!$D$30))
=IF(ISERROR('Week 1'!$D$39), "", ('Week 1'!$D$39))
=IF(ISERROR('Week 2'!$D$21), "", ('Week 2'!$D$21))
=IF(ISERROR('Week 2'!$D$30), "", ('Week 2'!$D$30))
=IF(ISERROR('Week 2'!$D$39), "", ('Week 2'!$D$39))

Without having to change each one by hand?

There is a pattern to the numbers.
There are 5 weeks, rows 12, 21, 30, 39, 48 in columns D,I,N,S,X

Thanks!
~Lizabeta