Allow null value for insert and updation

There are two posibilities.
1. your column is set as NOT ALLOW NULL
2. your field value is null and your are consider that column in INSERT QUERY

Make your column as ALLOW NULL and check field value for NULL before insertion.
if field value is NULL, Do not consider that column in INSET query. Database will automatically put NULL in that column.

you can ALLOW NULL your column by

ALTER TABLE TABLENAME ADD COLUMNNAME text NULL

10 tricks for handling null values in microsoft access[^]

Set the properties Of Access Column "Allow Null" to Yes....And show the error if this solution is not work....:)