Fix a possible double free in StopWatch::Refresh.

The lv_obj_del is called on btnStopLap  when transitioning to the
initial state, however the variable isn't then set to null. A subsequent
call to Refresh would attempt to delete the already freed object. This
could be triggered by stopping the stop watch, then pressing the
physical button on the watch.

Fixes https://github.com/JF002/InfiniTime/issues/315

src/displayapp/screens/StopWatch.cpp

@@ -115,8 +115,9 @@ bool StopWatch::Refresh() {
     // Init state when an user first opens the app
     // and when a stop/reset button is pressed
     case States::Init: {
-      if (btnStopLap) {
+      if (btnStopLap != nullptr) {
         lv_obj_del(btnStopLap);
+        btnStopLap = nullptr;
       }
       // The initial default value
       lv_label_set_text(time, "00:00");