java.lang.AssertionError: expected:<6> but was:<null>

at org.junit.Assert.fail(Assert.java:88)

at org.junit.Assert.failNotEquals(Assert.java:834)

at org.junit.Assert.assertEquals(Assert.java:118)

at org.junit.Assert.assertEquals(Assert.java:144)

at com.example.calculatortest.ExampleInstrumentedTest.testAdd(ExampleInstrumentedTest.java:54)

at java.lang.reflect.Method.invoke(Native Method)

at java.lang.reflect.Method.invoke(Method.java:372)

at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)

at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)

at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)

at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)

at android.support.test.internal.runner.junit4.statement.RunBefores.evaluate(RunBefores.java:80)

at android.support.test.internal.runner.junit4.statement.RunAfters.evaluate(RunAfters.java:61)

at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)

at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)

at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)

at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)

at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)

at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)

at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)

at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)

at org.junit.runners.ParentRunner.run(ParentRunner.java:363)

at android.support.test.runner.AndroidJUnit4.run(AndroidJUnit4.java:101)

at org.junit.runners.Suite.runChild(Suite.java:128)

at org.junit.runners.Suite.runChild(Suite.java:27)

at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)

at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)

at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)

at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)

at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)

at org.junit.runners.ParentRunner.run(ParentRunner.java:363)

at org.junit.runner.JUnitCore.run(JUnitCore.java:137)

at org.junit.runner.JUnitCore.run(JUnitCore.java:115)

at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:56)

at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:384)

at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1853)