android-首次在native页面调用driver.contexts，会导致之后异常

• Macaca Version:2.0.9

• Macaca Doctor Logs: macaca-doctor version: 2.0.2

  Node.js checklist:

  node env: /usr/local/bin/node node version: v6.10.2

  iOS checklist:

  Xcode Command Line Tools is ready, version: 2347. xcodebuild version: 8.3.3 iproxy[usbmuxd] is installed at: /usr/local/bin/iproxy ios_webkit_debug_proxy is installed at: /usr/local/bin/ios_webkit_debug_proxy carthage is installed, version: 0.23.0

  Android checklist:

  JAVA version is 1.8.0_121 JAVA_HOME is set to /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home ANDROID_HOME is set to /Users/joko/Library/Android/sdk Platforms is set to /Users/joko/Library/Android/sdk/platforms/android-25 ADB tool is set to /Users/joko/Library/Android/sdk/platform-tools/adb gradle is installed, version: 3.5

  Installed driver list:

  android: 2.0.36 electron: 1.1.6 [out-of-date] ios: 2.0.23 [out-of-date]

• Operate System: mac os 10.12.5

• Programming Language: python 2.7

预置环境:android 5.0 webview版本：54.0.2840.85 步骤： 1.使用macaca进行测试，停留在native界面调用driver.contexts方法，报错消息

Error: None
Message: no such session
  (Driver info: chromedriver=2.27.440174 (e97a722caafc2d3a8b807ee115bfb307f7d2cfd9),platform=Mac OS X 10.12.5 x86_64)

2.此时进入native或者webview页面调用driver.contexts，均会报以上错误 3.如果首次在webview页面调用driver.contexts方法，可正常打印上下文数据，之后再进入任何页面，均可正常打印

刷新页面
[u'NATIVE_APP', u'CDwindow-C16FA52D-6D9C-A6BC-7723-B8141C0F49CF']
刷新页面
[u'NATIVE_APP']
刷新页面
[u'NATIVE_APP', u'CDwindow-7809B59E-5612-1030-BC4A-B6F3768BF95D']
刷新页面

问题描述：首次调用场景会导致driver.contexts之后的异常，请帮忙修复此问题