android - NullPointer Error on running uiautomator test case on emulator -

i succeeded running first uiautomator test case in emulator, , goes fine. when started same test case second time in same emulator. got error:

instrumentation_status: stack=java.lang.nullpointerexception @ com.android.uiautomator.core.shelluiautomatorbridge.getdefaultdisplay(shelluiautomatorbridge.java:50) @ com.android.uiautomator.core.uidevice.getdisplaywidth(uidevice.java:378) @ com.android.uiautomator.core.uidevice.click(uidevice.java:408) @ test.uiautomator.apitest.testcase(apitest.java:172) @ java.lang.reflect.method.invokenative(native method) @ com.android.uiautomator.testrunner.uiautomatortestrunner.start(uiautomatortestrunner.java:160) @ com.android.uiautomator.testrunner.uiautomatortestrunner.run(uiautomatortestrunner.java:96) @ com.android.commands.uiautomator.runtestcommand.run(runtestcommand.java:91) @ com.android.commands.uiautomator.launcher.main(launcher.java:83) @ com.android.internal.os.runtimeinit.nativefinishinit(native method) @ com.android.internal.os.runtimeinit.main(runtimeinit.java:243) @ dalvik.system.nativestart.main(native method) 

does 1 knows reason? did not modify line in test case, ran second time.

i checked code , error occur here:

public display getdefaultdisplay() {     return  displaymanagerglobal.getinstance().getrealdisplay(display.default_display);// nullpointer here.  } 

it may becasue displaymanagerglobal.getinstance() returned null.

see code :

/**  * gets instance of display manager global singleton.  *  * @return display manager instance, may null in system startup  * before display manager has been initialized.  */ public static displaymanagerglobal getinstance() {     synchronized (displaymanagerglobal.class) {         if (sinstance == null) {             ibinder b = servicemanager.getservice(context.display_service);             if (b != null) {                 sinstance = new displaymanagerglobal(idisplaymanager.stub.asinterface(b));             }         }         return sinstance;     } }