问题
This only happends on the AVD devices within the AVD manager. I never get this error when capturing uiautomatorviewer screenshots from Genymotion.
When attempting to capture within the Android Device Monitor in AS...it gives me a little more detail: Reason: Error obtaining UI hierarchy
回答1:
Try it with HAXM acceleration turned off, if you can. I had to run the test using a profile that then ran the test on a physical device to get the screen shots. Essentially, if the AVD has Use GPU or Snapshot selected - even if running on a USB attached device, screenshots fail. This is a bug in uiautomator according to the developers involved with maturing the Appium platform.
回答2:
In addition to answer given by Houston Haynes, the problem can happen if you have set the flags for your activity like this:
getWindow().setFlags(WindowManager.LayoutParams.FLAG_SECURE, WindowManager.LayoutParams.FLAG_SECURE);
You can just comment the code and take the screenshots via anything(Android Studio, uiatutomatorviewer etc). Hope it helps!!
来源：https://stackoverflow.com/questions/25818574/ui-automator-viewer-receiving-error-taking-device-screenshot-null-every-time