Try:
Dim Rst As DAO.Recordset
Dim f As DAO.Field
Dim db As DAO.Database
Dim CReq As Boolean
Set db = CurrentDb
Set Rst = db.OpenRecordset("Table1", dbOpenTable, dbOpenSnapshot)
With Rst
Do While Not .EOF
CReq = False
For Each f In .Fields
If IsNull(f.Value) Then
Me.text1 = Me.text1 & IIf(CReq, ", ", "") & f.Name
CReq = True
End If
Next
If CReq THEN
Me.text1 = Me.text1 & vbCrLf
End If
.MoveNext
Loop
End With
Rst.Close