Update WorkoutActivity.java

app/src/main/java/be/lepl1509group13/workoutwarrior/WorkoutActivity.java

@@ -178,7 +178,7 @@ public class WorkoutActivity extends AppCompatActivity {
                     else {
                         set_countDownTimer.start();
                         set_break_timer_counting = true;
-                        if (workout_timer_counting) {
+                        if (workout_timer_counting && countdown != null) {
                             countdown.cancel();
                             workout_timer_counting = false;
                         }