fix(flex): fix crash with flex_grow on screen (#9090)

src/layouts/flex/lv_flex.c

@@ -116,7 +116,8 @@ void lv_obj_set_flex_align(lv_obj_t * obj, lv_flex_align_t main_place, lv_flex_a
 void lv_obj_set_flex_grow(lv_obj_t * obj, uint8_t grow)
 {
     lv_obj_set_style_flex_grow(obj, grow, 0);
-    lv_obj_mark_layout_as_dirty(lv_obj_get_parent(obj));
+    lv_obj_t * parent = lv_obj_get_parent(obj);
+    if(parent) lv_obj_mark_layout_as_dirty(parent);
 }
 
 /**********************

tests/src/test_cases/test_align_flex.c

@@ -191,4 +191,11 @@ void test_flex_use_rtl(void)
     TEST_ASSERT_EQUAL_SCREENSHOT("flex_use_rtl.png");
 }
 
+
+void test_flex_grow_on_screen_should_not_crash(void)
+{
+    lv_obj_set_flex_grow(lv_screen_active(), 1);
+    lv_refr_now(NULL);
+}
+
 #endif