Inorder to overcome this, if you feel you may get null values for a particular tag which you may want to assign later, then use an if condition to set it to empty text if the value is null. In such cases, the tag is created with empty value, so you can use it for assigning later.
There are cases where a BPEL assign activity on a variable may fail(Selection Failure) after a XSLtransform activity on the same variable. Reason is that a transform activity doesn’t create empty tags if the value is null. Hence, the variable will be updated with some missing tags(for null values) after transformation, and when you try to assign to such a tag, you will get an error.