Fix sidebar form update. (#5779) #13092

Job	Run time
Core Basic (18.x)	6m 33s
Core Content (18.x)	3m 23s
Core Basic (20.x)	6m 28s
Core Content (20.x)	4m 23s
Core Control Panels (18.x)	4m 25s
Core Volto Slate (18.x)	7m 39s
Core Control Panels (20.x)	3m 57s
Core Volto Slate (20.x)	7m 12s
Core Basic - Plone 5 (18.x)	7m 20s
Core Blocks (18.x)	7m 32s
Core Blocks - Listing (18.x)	4m 59s
Core Blocks (20.x)	6m 51s
Coresandbox (18.x)	4m 2s
Core Blocks - Listing (20.x)	5m 10s
Project Generator (18.x)	5m 23s
Multilingual (18.x)	3m 24s
Multilingual in Seamless Mode (18.x)	3m 32s
Project Generator (20.x)	5m 7s
Seamless Mode (18.x)	7m 2s
Working Copy (18.x)	3m 15s
	1h 47m 37s

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