Add check for post existence in admin bar #29360

Job	Run time
Core running from src / MacOS	2m 3s
Core running from src / Linux	2m 12s
Core running from build / Linux	2m 16s
Core running from build / Windows	3m 57s
Core running from build / MacOS	2m 0s
Core running from src / Windows	3m 7s
Slack Notifications	0s
Failed workflow tasks	0s
	15m 35s

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