For a demo I was writing a workflow activity and got the error:
Unexpected exception from plug-in (Execute): WorkFlowComponents.DeleteUnwantedPotentialLeads: System.MissingMethodException: Method not found: '!!0[] System.Array.Empty()'.
It was happening becasue my assembly version was not supported by Dynamics 365. As per this document Dynamcis 365 supports .Net Framework 4.5.2.
Corrected target .Net framework, redeployed assembly and same code started working as expected.
Hope it will help.
Yawer Iqbal 