Some of the new 2020-21 waste categories exist in 2014-15 and earlier

[tstephen]

For example: ALT_WASTE_DISPOSAL_WEIGHT, WEEE_WEIGHT, OTHER_RECOVERY_WEIGHT.

This means that we try to calculate the emissions but as this was not anticipated there is an error storing the result.

Workaround, check both usage and emission are present before calculating.