I have been futzing around with this for ages, but then saw the fix that worked for me below! Removing HyperDock from the Security & Privacy Accessibility pane in System Preferences, then switching to the HyperDock preferences pane pops up the usual dialogue saying it wants to control your machine. Click allow, then switch back to the Security & Privacy Accessibility pane and tick the box and, hey presto, it works! Thank goodness!! I have been lost without this since upgrading to Mojave!!