When wget runs and the activity closes, this happens:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: eu.quelltext.wget, PID: 29013
    java.lang.RuntimeException: Unable to stop activity {eu.quelltext.wget/eu.quelltext.wget.activities.CommandActivity}: java.lang.IllegalMonitorStateException: object not locked by thread before wait()
        at android.app.ActivityThread.callActivityOnStop(ActivityThread.java:4189)
        at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:4159)
        at android.app.ActivityThread.handleStopActivity(ActivityThread.java:4234)
        at android.app.servertransaction.StopActivityItem.execute(StopActivityItem.java:41)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6718)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.IllegalMonitorStateException: object not locked by thread before wait()
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:422)
        at eu.quelltext.wget.bin.Executable$ExecutionResult.kill(Executable.java:109)
        at eu.quelltext.wget.activities.CommandActivity.onStop(CommandActivity.java:201)
        at android.app.Instrumentation.callActivityOnStop(Instrumentation.java:1432)
        at android.app.Activity.performStop(Activity.java:7375)

Code:

        @Override
        public void kill() {
            process.destroy();
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                try {
                    process.wait(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                process.destroyForcibly();
            }
        }