ReflectionException error on Magento 2.1.x when running bin/magento setup:di:compile

[ReflectionException]
Class Magento\Framework\Api\SearchCriteria\CollectionProcessorInterface does not exist

This error is related to the WeSupply Toolbox module, and only shows up on Magento 2.1.x when running php bin/magento setup:di:compile

If you receive this error on Magento 2.1.x, there are two solutions:

Solution 1:

If you don't use the WeSupply Toolbox module, remove the whole WeSupply/Toolbox directory under app/code. 

Note: Simply disabling the extension via bin/magento module:disable is not sufficient. The files need to be removed completely. 

Solution 2:

If you need to use the WeSupply Toolbox module, please upgrade your Magento version to 2.2.x or 2.3.x, as this error only shows on Magento 2.1.x.