import android.app.TimePickerDialog;
import android.support.v4.app.DialogFragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
import android.widget.TimePicker;

    public class ButtonListeners extends FragmentActivity implements
View.OnClickListener, TimePickerDialog.OnTimeSetListener {

    @Override
    public void onClick(View buttons) {
        switch (buttons.getId()){
            case R.id.button1:
                break;
            case R.id.button2:
                DialogFragment timePicker = new TimePickerFragment();
                **FragmentManager manager = getSupportFragmentManager();** //<< error here
                timePicker.show(manager,"timepicker");

        }
    }

@Override
public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
      //do something
    }
}

public class TimePickerFragment extends DialogFragment {
    @NonNull
    @Override
    public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
        Calendar c = Calendar.getInstance();
        int hour = c.get(Calendar.HOUR_OF_DAY);
        int minute = c.get(Calendar.MINUTE);
        return new TimePickerDialog(getActivity(), (TimePickerDialog.OnTimeSetListener) getActivity(), hour, minute, DateFormat.is24HourFormat(getActivity()));
    }
}

2019-06-18 16:29:24.449 25857-25857/com.example.calendar E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.calendar, PID: 25857
java.lang.IllegalStateException: Activity has been destroyed
    at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:2114)
    at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:683)
    at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:637)
    at android.support.v4.app.DialogFragment.show(DialogFragment.java:144)
    at com.example.calendar.ButtonListeners.onClick(ButtonListeners.java:28)
    at android.view.View.performClick(View.java:6597)
    at android.view.View.performClickInternal(View.java:6574)
    at android.view.View.access$3100(View.java:778)
    at android.view.View$PerformClick.run(View.java:25885)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    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)