Flaky test:YarnSchedulerBackendSuite."RequestExecutors reflects node blacklist and is serializable"

Error Message
org.mockito.exceptions.misusing.WrongTypeOfReturnValue:  EmptySet$ cannot be returned by resourceOffers() resourceOffers() should return Seq *** If you're unsure why you're getting above error read on. Due to the nature of the syntax above problem might occur because: 1. This exception *might* occur in wrongly written multi-threaded tests.    Please refer to Mockito FAQ on limitations of concurrency testing. 2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies -     - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method. 
Stacktrace
sbt.ForkMain$ForkError: org.mockito.exceptions.misusing.WrongTypeOfReturnValue: 
EmptySet$ cannot be returned by resourceOffers()
resourceOffers() should return Seq
***
If you're unsure why you're getting above error read on.
Due to the nature of the syntax above problem might occur because:
1. This exception *might* occur in wrongly written multi-threaded tests.
   Please refer to Mockito FAQ on limitations of concurrency testing.
2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies - 
   - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.