I can reliably crash plasmashell as follows: Make sure the touchpad is on the "Status & Notifications" panel. Select Touchpad settings. Assign a "Meta+Shift+T" as the shortcut for Touchpad. Press "Ok". Open the "Status & Notifications" panel. Press Meta+Shift+T to activate the Touchpad widget. The "Status & Notifications" panel closes and then plasmashell crashes. Normally, Meta+Shift+T would open the "Status & Notifications" panel in order to show the Touchpad widget. The same procedure works with the "Notifications" widget, the "Device Notifier" widget and the "Printers" widget. I am using the Ubuntu package version "4:5.5.5.2-0ubuntu1". The --version option reports "plasmashell 5.5.5".
doens't seem to happen anymore