Why do donors see a "Attempt to de-reference a null object (sbxd)" error after making a recurring donation

If your donors receive the following error after submitting a recurring donation using the Donations app with the Salesforce integration turned on for your Donations app page...

Salesforce Error: sbxd.sbx_DonationsCampaignMember: execution of AfterInsert caused by: System.DmlException: ConvertLead failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, There was an error converting the lead. Please resolve the following error and try again: sbxd.sbx_DonationsLead: execution of AfterUpdate caused by: System.NullPointerException: Attempt to de-reference a null object (sbxd): [] (sbxd)

...ensure the following are correctly configured in your Salesforce org.

1) Confirm that all Lead mapping is completed correctly

Follow the instructions in the Installation Instructions for Soapbox Donations for Salesforce guide to ensure that all mapping of fields from the Lead object to the Account, Contact and Opportunity objects are completed