How to center buttons of flex column (v8.0)

Description

how to center buttons of flex column?
I tried align functions. lv_obj_center(obj); and lv_obj_align(obj, LV_ALIGN_CENTER, 0, 0);
but it did not work.
KakaoTalk_20210614_113542033

What MCU/Processor/Board and compiler are you using?

What LVGL version are you using?

What do you want to achieve?

What have you tried so far?

Code to reproduce

Add a code snippet which can run in the simulator. It should contain only the relevant code that compiles without errors when separated from your main code base.

The code block(s) should be formatted like:

	for(int i = 0; i < 10; i++)
	{
		lv_obj_t * obj;
		lv_obj_t * label;

		obj = lv_btn_create(cont_row);
		lv_obj_set_size(obj, 100, LV_PCT(100));

		label = lv_label_create(obj);
		lv_label_set_text_fmt(label, "Item: %d", i);
		lv_obj_center(label);

		obj = lv_btn_create(cont_col);
		lv_obj_set_size(obj, LV_PCT(50), LV_SIZE_CONTENT);
//		lv_obj_center(obj);
		lv_obj_align(obj, LV_ALIGN_CENTER, 0, 0);
		

		label = lv_label_create(obj);
		lv_label_set_text_fmt(label, "Item: %d", i);
		lv_obj_center(label);
		lv_obj_add_style(label, &test_style, LV_PART_MAIN | LV_STATE_DEFAULT);
	}

Screenshot and/or video

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