Android Failed to install HelloWorld.apk on device (null) Error
Try changing the ADB connection timeout. I think it defaults that to 5000ms and I changed mine to 10000ms to get rid of that problem. If you are in Eclipse, you can do this by going through Window -> Preferences and then it is in DDMS under Android.
As described here: Android error: Failed to install *.apk on device *: timeout
Restarting the device works for me. Using adb install can get the apk installed, but it's annoying to use it everytime you launch the app when debugging within eclipse.
I get this from time to time, but it's usually related to the emulator being slow to start. Try again without closing the emulator between the retries. And if it still fails, please post the client logs (logcat).
Another reason can be a ghost Eclipse process running in background and still bound to the debugging port. Close eclipse, look at process list and see if there's still an Eclipse running. Kill all of them and restart Eclipse again.