Added guard to check existence of callbackContext to prevent NullPointerException

[tomkelsey]

Hi guys,

I was intermittently getting an error on an Android device that caused the whole app to crash:

Shutting down VM E/AndroidRuntime(31057): FATAL EXCEPTION: main E/AndroidRuntime(31057): Process: net.memberoo.mobileapp, PID: 31057 E/AndroidRuntime(31057): java.lang.NullPointerException: Attempt to invoke virtual method 'void org.apache.cordova.CallbackContext.sendPluginResult(org.apache.cordova.PluginResult)' on a null object reference E/AndroidRuntime(31057): at com.evothings.BLE.keepCallback(BLE.java:164) E/AndroidRuntime(31057): at com.evothings.BLE.onLeScan(BLE.java:207) E/AndroidRuntime(31057): at android.bluetooth.BluetoothAdapter$3.onScanResult(BluetoothAdapter.java:2406) E/AndroidRuntime(31057): at android.bluetooth.le.BluetoothLeScanner$BleScanCallbackWrapper$1.run(BluetoothLeScanner.java:393) E/AndroidRuntime(31057): at android.os.Handler.handleCallback(Handler.java:739) E/AndroidRuntime(31057): at android.os.Handler.dispatchMessage(Handler.java:95) E/AndroidRuntime(31057): at android.os.Looper.loop(Looper.java:145) E/AndroidRuntime(31057): at android.app.ActivityThread.main(ActivityThread.java:6837) E/AndroidRuntime(31057): at java.lang.reflect.Method.invoke(Native Method) E/AndroidRuntime(31057): at java.lang.reflect.Method.invoke(Method.java:372) E/AndroidRuntime(31057): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) E/AndroidRuntime(31057): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) V/ApplicationPolicy( 3516): isApplicationStateBlocked userId 0 pkgname net.memberoo.mobileapp W/ActivityManager( 3516): Force finishing activity 1 net.memberoo.mobileapp/.MainActivity D/FocusedStackFrame( 3516): Set to : 0

I've added in a few guards to check that callbackContext isn't null and it seems to stop the app crashing.

[ghost]

@tomkelsey Thanks for the pull request. I will look further into this problem. This is caused by a bug somewhere else.