If you can´t edit Modern Design Pages or you can´t add News Posts in SharePoint 2019 Modern Design after Updating to the October 2020 Cumulative-Update (CU). Try adding Safe Controls in your web.config to resolve the Error.
My Customers also get Errors like “Unable to get property ‘call’ of undefined or null reference” from the sp-pages-assembly.js.
The fix was to add the layouts Folder to the Safe Controls:
<SafeControl Src="~/_layouts/*" IncludeSubFolders="True" Safe="True" AllowRemoteDesigner="True" SafeAgainstScript="True" />