WordPress 5.8 introduced a new block-based editor for Widgets.
Currently Toolset Blocks are not supported in the new editor.
Problems may include PHP warnings or notices in the logs, missing block styles on the front end, or block errors when adding a View block to widgets.
We are currently evaluating whether to add support for block-based widgets.
Classic Widgets is a popular plugin to disable this feature: https://wordpress.org/plugins/classic-widgets/