09-21 17:13:18.934 3851-3851/alura.daniel.jumper E/AndroidRuntime: FATAL EXCEPTION: main
                                                                   Process: alura.daniel.jumper, PID: 3851
                                                                   java.lang.RuntimeException: Unable to resume activity {alura.daniel.jumper/alura.daniel.jumper.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void alura.daniel.jumper.engine.Game.inicia()' on a null object reference
                                                                       at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3121)
                                                                       at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3152)
                                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2495)
                                                                       at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
                                                                       at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                       at android.os.Looper.loop(Looper.java:148)
                                                                       at android.app.ActivityThread.main(ActivityThread.java:5443)
                                                                       at java.lang.reflect.Method.invoke(Native Method)
                                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
                                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
                                                                    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void alura.daniel.jumper.engine.Game.inicia()' on a null object reference
                                                                       at alura.daniel.jumper.MainActivity.onResume(MainActivity.java:29)
                                                                       at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1281)
                                                                       at android.app.Activity.performResume(Activity.java:6320)
                                                                       at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3110)
                                                                       at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3152) 
                                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2495) 
                                                                       at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354) 
                                                                       at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                       at android.os.Looper.loop(Looper.java:148) 
                                                                       at android.app.ActivityThread.main(ActivityThread.java:5443) 
                                                                       at java.lang.reflect.Method.invoke(Native Method) 
                                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) 
                                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

Esse é o erro que aparece no Android Monitor

Como resolver?

Os métodos inicia() e onResume() estão assim:

    public void inicia() {
        isRunning = true;
    }

protected void onResume() {
        super.onResume();
        game.inicia();
        new Thread(game).start();
    }