Error - 'ddlCustomerCurrencyy has a SelectedValue which is invalid...' when trying to change the customer on a fee note via the project record

This error will occur if the currency used on the transaction no longer exists or if the date range does not match when using date sensitive rates - usually this will occur on an Invoice which has been reopened.
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Please ensure that there is a currency with the same code as on the transaction and that the date range matches. The transaction should then no longer error.