I have a reproducible crash in Reaper v5/pre22. Win 7 x64
It happens when Playtime is docked and where there is an empty area of the dock that the Playtime GUI doesn't take up. So on my system I have Playtime docked at the bottom left and the Media Explorer in the bottom right, expanding the dock vertically leaves empty space below the Playtime GUI.
To make it happen, right-click on the Playtime GUI to bring up the context menu then - leaving it open - right-click on the empty area of the dock it's in.