Testing DoFn Apache Crunch

Question

I am very new to Apache Crunch. This is the first test case I have written. Currently I am writing test cases for DoFn but it says NullPointerException.

import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.verifyNoMoreInteractions;
import org.apache.crunch.Emitter;
import org.apache.crunch.Pair;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class OutputResultTest {

  @Mock
  InMemoryEmitter <String>  emitter = new InMemoryEmitter<>();

  @Test
  public void TestProcess(){
    OutputResult outputResult = new OutputResult();
    //String expected = "Thor Odinson scored 97.8";
    Pair<String,Double> input = Pair.of("Thor Odinson",97.8);
    outputResult.process(input,emitter);
    assertEquals(expected,emitter.getOutput());
    //verify(emitter).emit("Thor Odinson scored 97.8");//this works
    //verifyNoMoreInteractions(emitter);//this works
  }
}