vba code if lastrow is full, it errors out

[chubbychub]

i have userform with a vba code, the vba code its supposed to fill from inputbox 'boxestotal' into the next available empty cell within a range of cell on column d, but if cell D25 if full (first) then it detects it as entire D5 to D25 full, (altho there are empty cellss in between.
attached is a sample workbook. Please help, thanks in advance

PS. I used D25 to fill some data first sometimes.