Symptoms

After upgrading the Staging environment to 4.7, some workflows will open in K2 Studio while others will error with:

Value cannot be null. Parameter name: field
 

Diagnoses

It was determined that K2 Studio was pointing toward the Production K2 Server to retrieve environment libraries this server had not yet been upgraded to 4.7. As such, it was likely a due to the difference in K2 component version and perhaps an environment library field that was used in one workflow and not the other.
 

Resolution

Updating K2 Studio to point toward the 4.7 Staging server via (File > Options) and restarting K2 Studio resolved this issue.