Error on sign out: IllegalStateException: onSubscribe not called in proper order

[dnmolchanov]

How to reproduce:
Add the following line to the signOut() test in RxFirebaseAuthTest.kt:938: assertNoErrors() and run the test.
Result:

java.lang.AssertionError: Error(s) present: [java.lang.IllegalStateException: onSubscribe not called in proper order] (latch = 0, values = 0, errors = 1, completions = 1)

	at io.reactivex.observers.BaseTestConsumer.fail(BaseTestConsumer.java:191)
	at io.reactivex.observers.BaseTestConsumer.assertNoErrors(BaseTestConsumer.java:276)
	at com.androidhuman.rxfirebase2.auth.RxFirebaseAuthTest.signOut(RxFirebaseAuthTest.kt:938)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	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.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	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$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$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)
Caused by: java.lang.IllegalStateException: onSubscribe not called in proper order
	at io.reactivex.observers.TestObserver.onComplete(TestObserver.java:187)
	at com.androidhuman.rxfirebase2.auth.SignOutObserver.subscribeActual(SignOutObserver.java:19)
	at io.reactivex.Completable.subscribe(Completable.java:1741)
	at com.androidhuman.rxfirebase2.auth.RxFirebaseAuthTest.signOut(RxFirebaseAuthTest.kt:933)
	... 23 more


Process finished with exit code 255