Fatal Exception: java.lang.NullPointerException

**Conductor version:**
implementation 'com.bluelinelabs:conductor:3.0.0-rc6'

**API level:**
Android 5, 6, 7, 8, 9, 10

**Error log:**
```
Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.animation.Animator.addListener(android.animation.Animator$AnimatorListener)' on a null object reference
at com.bluelinelabs.conductor.changehandler.AnimatorChangeHandler.performAnimation(AnimatorChangeHandler.java:182)
at com.bluelinelabs.conductor.changehandler.AnimatorChangeHandler.performChange(AnimatorChangeHandler.java:139)
at com.bluelinelabs.conductor.ControllerChangeHandler.executeChange(ControllerChangeHandler.java:209)
at com.bluelinelabs.conductor.ControllerChangeHandler.executeChange(ControllerChangeHandler.java:159)
at com.bluelinelabs.conductor.Router.performControllerChange(Router.java:834)
at com.bluelinelabs.conductor.Router.performControllerChange(Router.java:799)
at com.bluelinelabs.conductor.Router.performControllerChange(Router.java:777)
at com.bluelinelabs.conductor.Router.popController(Router.java:137)
at com.bluelinelabs.conductor.Router.popCurrentController(Router.java:119)
at com.bluelinelabs.conductor.Router.handleBack(Router.java:97)
at com.example.MainActivity.onBackPressed(MainActivity.java:269)
at android.app.Activity.onKeyUp(Activity.java:2864)
at android.view.KeyEvent.dispatch(KeyEvent.java:3358)
at androidx.core.view.KeyEventDispatcher.activitySuperDispatchKeyEventPre28(KeyEventDispatcher.java:137)
at androidx.core.view.KeyEventDispatcher.dispatchKeyEvent(KeyEventDispatcher.java:87)
at androidx.core.app.ComponentActivity.dispatchKeyEvent(ComponentActivity.java:140)
at androidx.appcompat.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:569)
at androidx.appcompat.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:59)
at androidx.appcompat.app.AppCompatDelegateImpl$AppCompatWindowCallback.dispatchKeyEvent(AppCompatDelegateImpl.java:3054)
at androidx.appcompat.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:59)
at com.android.internal.policy.DecorView.dispatchKeyEvent(DecorView.java:428)
at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:5007)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4969)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4504)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4557)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4523)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4656)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4531)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4713)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4504)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4557)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4523)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4531)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4504)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4557)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4523)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4689)
at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:4858)
at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:3036)
at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:2574)
at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:2565)
at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:3013)
at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141)
at android.os.MessageQueue.nativePollOnce(MessageQueue.java)
at android.os.MessageQueue.next(MessageQueue.java:323)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:6682)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)
```

The stack trace points to AnimatorChangeHandler.performAnimation at line 182, reached through ControllerChangeHandler and Router.popController. Start by inspecting how the animator is created before addListener and reproduce the back-navigation path on the listed Android API levels. Done means the reported null animator crash is prevented and the affected behavior is covered by a regression test.

Written by the indexing model from the issue text.