As TestNG interleaves tests from different classes (even in single-threaded mode), cleanup of one class (@AfterClass) can happen to be invoked in the middle of executing tests from another class. More precisely, it clears the Container instance:

at org.jboss.weld.Container.cleanup(Container.java:108)
at org.jboss.weld.bootstrap.WeldBootstrap.shutdown(WeldBootstrap.java:460)
at org.jboss.arquillian.container.weld.ee.embedded_1_1.mock.MockServletLifecycle.endApplication(MockServletLifecycle.java:126)
at org.jboss.arquillian.container.weld.ee.embedded_1_1.mock.TestContainer.stopContainer(TestContainer.java:139)
at org.jboss.arquillian.container.weld.ee.embedded_1_1.WeldEEMockContainer.undeploy(WeldEEMockContainer.java:131)
at org.jboss.arquillian.impl.handler.ContainerUndeployer.callback(ContainerUndeployer.java:61)
at org.jboss.arquillian.impl.handler.ContainerUndeployer.callback(ContainerUndeployer.java:47)
at org.jboss.arquillian.impl.event.MapEventManager.fire(MapEventManager.java:63)
at org.jboss.arquillian.impl.context.AbstractEventContext.fire(AbstractEventContext.java:115)
at org.jboss.arquillian.impl.EventTestRunnerAdaptor.afterClass(EventTestRunnerAdaptor.java:86)
at org.jboss.arquillian.testng.Arquillian.arquillianAfterClass(Arquillian.java:83)
at sun.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-1)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.testng.internal.MethodHelper.invokeMethod(MethodHelper.java:644)
at org.testng.internal.Invoker.invokeConfigurationMethod(Invoker.java:443)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:160)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:90)
at org.testng.internal.TestMethodWorker.invokeAfterClassMethods(TestMethodWorker.java:227)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:124)
at org.testng.TestRunner.runWorkers(TestRunner.java:908)
at org.testng.TestRunner.privateRun(TestRunner.java:617)
at org.testng.TestRunner.run(TestRunner.java:498)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:329)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:324)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:296)
at org.testng.SuiteRunner.run(SuiteRunner.java:201)
at org.testng.TestNG.createAndRunSuiteRunners(TestNG.java:915)
at org.testng.TestNG.runSuitesLocally(TestNG.java:879)
at org.testng.TestNG.run(TestNG.java:787)
at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:75)
at org.testng.RemoteTestNGStarter.main(RemoteTestNGStarter.java:92)

(notice the AfterClass, Arquillian:83), which causes any subsequent test to throw:

org.jboss.arquillian.impl.event.FiredEventException: java.lang.IllegalStateException: Singleton is not set
at org.jboss.arquillian.impl.event.MapEventManager.fire(MapEventManager.java:68)
at org.jboss.arquillian.impl.context.AbstractEventContext.fire(AbstractEventContext.java:115)
at org.jboss.arquillian.impl.context.AbstractEventContext.fire(AbstractEventContext.java:113)
at org.jboss.arquillian.impl.EventTestRunnerAdaptor.before(EventTestRunnerAdaptor.java:97)
at org.jboss.arquillian.testng.Arquillian.arquillianBeforeTest(Arquillian.java:89)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.testng.internal.MethodHelper.invokeMethod(MethodHelper.java:644)
at org.testng.internal.Invoker.invokeConfigurationMethod(Invoker.java:443)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:160)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:494)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:700)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1002)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:137)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:121)
at org.testng.TestRunner.runWorkers(TestRunner.java:908)
at org.testng.TestRunner.privateRun(TestRunner.java:617)
at org.testng.TestRunner.run(TestRunner.java:498)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:329)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:324)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:296)
at org.testng.SuiteRunner.run(SuiteRunner.java:201)
at org.testng.TestNG.createAndRunSuiteRunners(TestNG.java:915)
at org.testng.TestNG.runSuitesLocally(TestNG.java:879)
at org.testng.TestNG.run(TestNG.java:787)
at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:75)
at org.testng.RemoteTestNGStarter.main(RemoteTestNGStarter.java:92)
Caused by: java.lang.IllegalStateException: Singleton is not set
at org.jboss.weld.bootstrap.api.helpers.IsolatedStaticSingletonProvider$IsolatedStaticSingleton.get(IsolatedStaticSingletonProvider.java:52)
at org.jboss.weld.Container.instance(Container.java:58)
at org.jboss.weld.bean.proxy.ClientProxyMethodHandler.getProxiedInstance(ClientProxyMethodHandler.java:138)
at org.jboss.weld.bean.proxy.ClientProxyMethodHandler.invoke(ClientProxyMethodHandler.java:100)
at org.jboss.weld.util.CleanableMethodHandler.invoke(CleanableMethodHandler.java:43)
at org.jboss.weld.servlet.HttpSessionManager_$$javassist_14.setSession(HttpSessionManager$$_javassist_14.java)
at org.jboss.arquillian.container.weld.ee.embedded_1_1.WeldEEMockContainer$1.callback(WeldEEMockContainer.java:111)
at org.jboss.arquillian.container.weld.ee.embedded_1_1.WeldEEMockContainer$1.callback(WeldEEMockContainer.java:95)
at org.jboss.arquillian.impl.event.MapEventManager.fire(MapEventManager.java:63)
... 28 more