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
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.
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