Fix DumpWindowHierarchy NullPointerException

Fix #2655. It doesn't crash however this appears in the logs:

```
info: Pushing command to appium work queue: ["dumpWindowHierarchy"]
info: [BOOTSTRAP] [info] Got data from client: {"cmd":"action","action":"dumpWindowHierarchy","params":{}}
info: [BOOTSTRAP] [info] Got command of type ACTION
info: [BOOTSTRAP] [debug] Got command action: dumpWindowHierarchy
info: [UIAUTOMATOR STDOUT] java.lang.NullPointerException
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.childNafCheck(AccessibilityNodeInfoDumper.java:200)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.nafCheck(AccessibilityNodeInfoDumper.java:180)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.dumpNodeRec(AccessibilityNodeInfoDumper.java:104)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.dumpNodeRec(AccessibilityNodeInfoDumper.java:129)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.dumpNodeRec(AccessibilityNodeInfoDumper.java:129)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.dumpNodeRec(AccessibilityNodeInfoDumper.java:129)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.AccessibilityNodeInfoDumper.dumpWindowToFile(AccessibilityNodeInfoDumper.java:89)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.core.UiDevice.dumpWindowHierarchy(UiDevice.java:768)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.handler.DumpWindowHierarchy.dump(DumpWindowHierarchy.java:36)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.handler.DumpWindowHierarchy.execute(DumpWindowHierarchy.java:71)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.AndroidCommandExecutor.execute(AndroidCommandExecutor.java:94)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.SocketServer.runCommand(SocketServer.java:170)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.SocketServer.handleClientData(SocketServer.java:91)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.SocketServer.listenForever(SocketServer.java:135)
info: [UIAUTOMATOR STDOUT] at io.appium.android.bootstrap.Bootstrap.testRunServer(Bootstrap.java:17)
info: [UIAUTOMATOR STDOUT] at java.lang.reflect.Method.invokeNative(Native Method)
info: [UIAUTOMATOR STDOUT] at java.lang.reflect.Method.invoke(Method.java:515)
info: [UIAUTOMATOR STDOUT] at junit.framework.TestCase.runTest(TestCase.java:168)
info: [UIAUTOMATOR STDOUT] at junit.framework.TestCase.runBare(TestCase.java:134)
info: [UIAUTOMATOR STDOUT] at junit.framework.TestResult$1.protect(TestResult.java:115)
info: [UIAUTOMATOR STDOUT] at junit.framework.TestResult.runProtected(TestResult.java:133)
info: [UIAUTOMATOR STDOUT] at junit.framework.TestResult.run(TestResult.java:118)
info: [UIAUTOMATOR STDOUT] at junit.framework.TestCase.run(TestCase.java:124)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.testrunner.UiAutomatorTestRunner.start(UiAutomatorTestRunner.java:160)
info: [UIAUTOMATOR STDOUT] at com.android.uiautomator.testrunner.UiAutomatorTestRunner.run(UiAutomatorTestRunner.java:96)
info: [UIAUTOMATOR STDOUT] at com.android.commands.uiautomator.RunTestCommand.run(RunTestCommand.java:91)
info: [UIAUTOMATOR STDOUT] at com.android.commands.uiautomator.Launcher.main(Launcher.java:83)
info: [UIAUTOMATOR STDOUT] at com.android.internal.os.RuntimeInit.nativeFinishInit(Native Method)
info: [UIAUTOMATOR STDOUT] at com.android.internal.os.RuntimeInit.main(RuntimeInit.java:243)
info: [UIAUTOMATOR STDOUT] at dalvik.system.NativeStart.main(Native Method)
info: [BOOTSTRAP] [info] Returning result: {"value":true,"status":0}
debug: transferPageSourceXML command: "/Users/user/woven/android-sdk-macosx/platform-tools/adb" -s emulator-5554 pull /data/local/tmp/dump.xml "/var/folders/w7/c1yh5bps5dnc0frz0tbj_dmh0000gn/T/114423-52462-h6wajo.xml"
invalid document source 
@#[line:0,col:undefined]
error: 
```

I'm not sure where this comes from:

```
invalid document source 
@#[line:0,col:undefined]
error: 
```