fix: fix null pointer dereference (#4155)

src/core/lv_refr.c

@@ -294,6 +294,11 @@ void _lv_disp_refr_timer(lv_timer_t * tmr)
         disp_refr = lv_disp_get_default();
     }
 
+    if(disp_refr == NULL) {
+        LV_LOG_WARN("No display registered");
+        return;
+    }
+
     if(disp_refr->draw_buf_size == 0) {
         LV_LOG_WARN("draw_buf_size == 0");
         return;

src/widgets/chart/lv_chart.c

@@ -696,6 +696,7 @@ static void lv_chart_destructor(const lv_obj_class_t * class_p, lv_obj_t * obj)
     lv_chart_series_t * ser;
     while(chart->series_ll.head) {
         ser = _lv_ll_get_head(&chart->series_ll);
+        if(!ser) continue;
 
         if(!ser->y_ext_buf_assigned) lv_free(ser->y_points);