Robolectric中的NotificationManager.isNotificationPolicyAccessGranted NPE

Question

我想写一个涉及Robolectric notificationManager.isNotificationPolicyAccessGranted的简单测试。

package com.xi_zz.mutephonebutton

import android.app.NotificationManager
import android.app.NotificationManager.INTERRUPTION_FILTER_ALL
import android.app.NotificationManager.INTERRUPTION_FILTER_PRIORITY
import android.content.Context
import org.junit.Assert.assertEquals
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.robolectric.RobolectricTestRunner
import org.robolectric.RuntimeEnvironment


@RunWith(RobolectricTestRunner::class)
    class MainViewModelTest {
        lateinit var notificationManager: NotificationManager

        @Before
        fun setUp() {
            notificationManager = RuntimeEnvironment.application.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        }

        @Test
        fun testTest() {
            assertEquals(notificationManager.getCurrentInterruptionFilter(), INTERRUPTION_FILTER_ALL); // Pass
            notificationManager.setInterruptionFilter(INTERRUPTION_FILTER_PRIORITY);
            assertEquals(notificationManager.getCurrentInterruptionFilter(), INTERRUPTION_FILTER_PRIORITY); // Pass

            var bool = notificationManager.isNotificationPolicyAccessGranted // NPE
        }
    }

错误：

java.lang.NullPointerException
    at android.app.NotificationManager.isNotificationPolicyAccessGranted(NotificationManager.java:773)
    at com.xi_zz.mutephonebutton.MainViewModelTest.testTest(MainViewModelTest.kt:31)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.robolectric.RobolectricTestRunner$HelperTestRunner$1.evaluate(RobolectricTestRunner.java:570)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.robolectric.internal.SandboxTestRunner$2.evaluate(SandboxTestRunner.java:253)
    at org.robolectric.internal.SandboxTestRunner.runChild(SandboxTestRunner.java:130)
    at org.robolectric.internal.SandboxTestRunner.runChild(SandboxTestRunner.java:42)
    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.robolectric.internal.SandboxTestRunner$1.evaluate(SandboxTestRunner.java:84)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
    at com.intellij.rt.execution.application.AppMainV2.main(AppMainV2.java:131)

notificationManager一直有效，直到我拨打isNotificationPolicyAccessGranted。知道为什么会这样吗？我使用的是Robolectric 3.8和Junit 4.12。