You created your aggregate with the HANDLESNULLS option, but the function named in the error message does not handle nulls.
When you create an aggregate with HANDLESNULLS, all support functions must also handle nulls. You can either change the aggregate or change the function. To change the aggregate, drop the aggregate and re-create it without the HANDLESNULLS option.
If the aggregate needs to handle null values, you must modify the function so that it can handle nulls. Then drop the function and reregister it with the HANDLESNULLS option.