NullPointerException when there are racy resources

Only happened once, cannot reproduce, but reading the code confirms there's a bug here:

https://github.com/android/android-test/blob/eacf07d12d8ff3e2d5a1834f1f33d1e47d352de3/espresso/core/java/androidx/test/espresso/base/UiControllerImpl.java#L553-L559

https://github.com/android/android-test/blob/eacf07d12d8ff3e2d5a1834f1f33d1e47d352de3/espresso/core/java/androidx/test/espresso/util/StringJoiner.kt#L34

the above expects a non-null list, but the below returns null:

https://github.com/android/android-test/blob/eacf07d12d8ff3e2d5a1834f1f33d1e47d352de3/espresso/core/java/androidx/test/espresso/base/IdlingResourceRegistry.java#L366-L387

```
java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.test.espresso.util.StringJoinerKt.joinToString, parameter iterable
at androidx.test.espresso.Espresso.onIdle(Espresso.java:357)
at androidx.test.espresso.Espresso.onIdle(Espresso.java:378)
at androidx.compose.ui.test.EspressoLink_androidKt.runEspressoOnIdle(EspressoLink.android.kt:88)
at androidx.compose.ui.test.EspressoLink.runUntilIdle(EspressoLink.android.kt:82)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment.waitForIdle(ComposeUiTest.android.kt:596)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment.access$waitForIdle(ComposeUiTest.android.kt:391)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$AndroidTestOwner.getRoots(ComposeUiTest.android.kt:822)
at androidx.compose.ui.test.TestOwnerKt.getAllSemanticsNodes(TestOwner.kt:81)
at androidx.compose.ui.test.SemanticsNodeInteraction.fetchSemanticsNodes$ui_test_release(SemanticsNodeInteraction.kt:64)
at androidx.compose.ui.test.SemanticsNodeInteraction.fetchSemanticsNodes$ui_test_release$default(SemanticsNodeInteraction.kt:59)
at androidx.compose.ui.test.AndroidAssertions_androidKt.checkIsDisplayed(AndroidAssertions.android.kt:29)
at androidx.compose.ui.test.AssertionsKt.isDisplayed(Assertions.kt:351)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$AndroidComposeUiTestImpl.waitUntil(ComposeUiTest.android.kt:747)
at androidx.compose.ui.test.junit4.AndroidComposeTestRule.waitUntil(AndroidComposeTestRule.android.kt:362)
at com.example.MyTest.myTest(MyTest.kt:78)
at java.lang.reflect.Method.invoke(Native Method)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at androidx.test.internal.runner.junit4.statement.RunBefores.evaluate(RunBefores.java:80)
at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
at androidx.test.rule.GrantPermissionRule$RequestPermissionStatement.evaluate(GrantPermissionRule.java:150)
at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$testWithDisposal$1.evaluate(AndroidComposeTestRule.android.kt:306)
at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1$evaluate$1.invokeSuspend(AndroidComposeTestRule.android.kt:318)
at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1$evaluate$1.invoke(Unknown Source:8)
at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1$evaluate$1.invoke(Unknown Source:4)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1$1$1$1.invokeSuspend(ComposeUiTest.android.kt:569)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1$1$1$1.invoke(Unknown Source:8)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1$1$1$1.invoke(Unknown Source:2)
at androidx.compose.ui.test.EspressoLink.withStrategy(EspressoLink.android.kt:69)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1.invokeSuspend(ComposeUiTest.android.kt:566)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1.invoke(Unknown Source:8)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1.invoke(Unknown Source:4)
at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt$runTest$2$1$1.invokeSuspend(TestBuilders.kt:317)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
at kotlinx.coroutines.test.TestDispatcher.processEvent$kotlinx_coroutines_test(TestDispatcher.kt:24)
at kotlinx.coroutines.test.TestCoroutineScheduler.tryRunNextTaskUnless$kotlinx_coroutines_test(TestCoroutineScheduler.kt:99)
at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt$runTest$2$1$workRunner$1.invokeSuspend(TestBuilders.kt:326)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:263)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:94)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:70)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source:1)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:48)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source:1)
at kotlinx.coroutines.test.TestBuildersJvmKt.createTestResult(TestBuildersJvm.kt:10)
at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt.runTest-8Mi8wO0(TestBuilders.kt:309)
at kotlinx.coroutines.test.TestBuildersKt.runTest-8Mi8wO0(Unknown Source:1)
at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt.runTest-8Mi8wO0(TestBuilders.kt:167)
at kotlinx.coroutines.test.TestBuildersKt.runTest-8Mi8wO0(Unknown Source:1)
at androidx.compose.ui.test.AndroidComposeUiTestEnvironment.runTest(ComposeUiTest.android.kt:549)
at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1.evaluate(AndroidComposeTestRule.android.kt:318)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:68)
at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:59)
at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:467)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:2594)
Caused by: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.test.espresso.util.StringJoinerKt.joinToString, parameter iterable
at java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.util.concurrent.FutureTask.get(FutureTask.java:191)
at androidx.test.espresso.Espresso.onIdle(Espresso.java:349)
... 100 more
Caused by: java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.test.espresso.util.StringJoinerKt.joinToString, parameter iterable
at androidx.test.espresso.util.StringJoinerKt.joinToString(Unknown Source:2)
at androidx.test.espresso.base.UiControllerImpl.loopUntil(UiControllerImpl.java:559)
at androidx.test.espresso.base.UiControllerImpl.loopMainThreadUntilIdle(UiControllerImpl.java:434)
at androidx.test.espresso.Espresso.lambda$onIdle$2(Espresso.java:340)
at androidx.test.espresso.Espresso$$ExternalSyntheticLambda1.call(D8$$SyntheticClass:0)
at java.util.concurrent.FutureTask.run(FutureTask.java:264)
at android.os.Handler.handleCallback(Handler.java:959)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loopOnce(Looper.java:232)
at android.os.Looper.loop(Looper.java:317)
at android.app.ActivityThread.main(ActivityThread.java:8705)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:580)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:886)
```

Start with UiControllerImpl.java lines 553-559 and trace the value passed to StringJoiner.kt line 34. Then inspect IdlingResourceRegistry.java lines 366-387, focusing on the racy-resource path that can return null. Done means the idle loop no longer throws this NullPointerException when that race occurs, with a regression test if the relevant test location can be identified.

Written by the indexing model from the issue text.