In gnome-shell 3.30.1, when using this extension, I get a tiny grey box instead of workspace previews, and then gnome-shell restarts.
Oct 31 15:44:30 bilbo-Precision-5520 gnome-shell[21070]: Some code accessed the property 'ThumbnailsBox' on the module 'thumbnailsBox'. That property was defined with 'let' or 'const' inside the module. This was previously supported, but is not correct according to the ES6 standard. Any symbols to be exported from a module must be defined with 'var'. The property access will work as previously for the time being, but please fix your code anyway.
Oct 31 15:44:30 bilbo-Precision-5520 gnome-shell[21070]: JS ERROR: TypeError: thumbnail is undefined
_activeWorkspaceChanged@resource:///org/gnome/shell/ui/workspaceThumbnail.js:1347:28
wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
actionMoveWorkspace@resource:///org/gnome/shell/ui/windowManager.js:2125:13
wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
_showWorkspaceSwitcher@resource:///org/gnome/shell/ui/windowManager.js:2099:13
wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
Oct 31 15:44:31 bilbo-Precision-5520 gnome-shell[21070]: JS WARNING: [/home/bilbo/.local/share/gnome-shell/extensions/gnome-shell-extension-workspace-switcher-popup-thumbnails@sustmidown.centrum.cz/thumbnailsBox.js 95]: reference to undefined property "_signals"