fix(refr): fix performance monitor NULL pointer access (lvgl#3105)

* fix(refr) performance monitor NULL pointer access

* doc(refr): describe the setting of LV_DISP_DEF_REFR_PERIOD

Co-authored-by: pengyiqiang <[email protected]>

Author: FASTSHIFT

docs/porting/display.md

@@ -228,6 +228,8 @@ If you have multiple displays call `lv_disp_set_deafult(disp1);` to select the d
 
 Note that `lv_timer_handler()` and `_lv_disp_refr_timer()` can not run at the same time.
 
+If the performance monitor is enabled, the value of `LV_DISP_DEF_REFR_PERIOD` needs to be set to be consistent with the refresh period of the display to ensure that the statistical results are correct.
+
 ## Further reading
 
 - [lv_port_disp_template.c](https://github.com/lvgl/lvgl/blob/master/examples/porting/lv_port_disp_template.c) for a template for your own driver.

src/core/lv_refr.c

@@ -376,9 +376,16 @@ void _lv_disp_refr_timer(lv_timer_t * tmr)
     }
     else {
         perf_monitor.perf_last_time = lv_tick_get();
-        uint32_t fps_limit = 1000 / disp_refr->refr_timer->period;
+        uint32_t fps_limit;
         uint32_t fps;
 
+        if(disp_refr->refr_timer) {
+            fps_limit = 1000 / disp_refr->refr_timer->period;
+        }
+        else {
+            fps_limit = 1000 / LV_DISP_DEF_REFR_PERIOD;
+        }
+
         if(perf_monitor.elaps_sum == 0) {
             perf_monitor.elaps_sum = 1;
         }