fix(display): check NULL act_screen before using it (#4973)

Signed-off-by: Xu Xingliang <xuxingliang@xiaomi.com>
Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

tests/src/test_cases/test_screen_load.c

@@ -0,0 +1,15 @@
+#if LV_BUILD_TEST
+#include "../lvgl.h"
+
+#include "unity/unity.h"
+
+void test_screen_load_no_crash(void)
+{
+    /*Delete active screen and load new screen should not crash*/
+    lv_obj_t * screen = lv_screen_active();
+    lv_obj_del(screen);
+    screen = lv_obj_create(NULL);
+    lv_screen_load(screen);
+}
+
+#endif