Did you ever get this fixed? I am hitting the same problem - cannot test a view containing a slot.
Error: Unhandled promise rejection: Error invoking SlotCustomAttribute. Check the inner error for details.
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Inner Error:
Message: key/value cannot be null or undefined. Are you trying to inject/register something that doesn't exist with DI?
Inner Error Stack:
Error: key/value cannot be null or undefined. Are you trying to inject/register something that doesn't exist with DI?