How to unit test flink timers using mockito

Question

I have a process funtion, im using mockito for unit testing.

class SampleProcessFunction : KeyedProcessFunction<String, String, String>() {{
        override fun processElement(dm: String, ctx: Context, out: Collector<String>) {
            var retrieveState = state.value()
            // schedule the next timer  from the current event time
            ctx.timerService().registerEventTimeTimer(ctx.timestamp() + 3000)
        } 
    
     override fun onTimer(scheduledTimer: Long, ctx: OnTimerContext?, out: Collector<String>) {
            val retrieveState = state.value()
    
            if(scheduledTimer == (ctx.timestamp()+3000)) {
                         out.collect(presenceEvent)
            }
        }
}

I wanted to unit test the process function using mockito

 internal class PresenceDetectionProcessFunctionTest {
    
        private lateinit var out: Collector<String>
        private lateinit var mockPresenceState: ValueState<Presence>
    
    
        @BeforeEach
        fun setUp() {
            SampleProcessFunction = SampleProcessFunction()
            out = mock(Collector::class.java) as Collector<PresenceEvent>
            val mockRunTimeContext = mock(RuntimeContext::class.java)
            mockSampleState = mock(ValueState::class.java) as ValueState<SampleState>
           
    
            whenever(mockRunTimeContext.getState(ValueStateDescriptor("sample-state", 
             Sample::class.java))).thenReturn(mockSampleState)      
            
             sampleProcessFunction.runtimeContext = mockRunTimeContext
            sampleProcessFunction.open(Configuration())
        }
    
    @Test
    fun `test sample process function`() {
       
        sampleProcessFunction.processElement("Hellp, context, out)
        Mockito.verify(mockPresenceState, times(1)).update(result.capture())
       }

How do we test the timer here, in my process fun i have register the event timer, but when I debug throws NPE at ctx.timerService().registerEventTimeTimer(ctx.timestamp() + 3000) in sample process function as I did not setup any test configuration for timers in my test. How do we test the flink timer functionality here? Any suggestions?