You can fix this within the PHP code in the file:
/custom-tables/pages/setup_table.php : line 485
from:
document.forms[0].insertBefore(newFieldContainer,nFloor);
to:
document.getElementsByName('wctform')[0].insertBefore(newFieldContainer,nFloor);
This will target the form specifically (as faina09 mentioned above) and adds the extra fields.