If I knew what "FW..." stood for, then I might be cured!
More: This tool is very fussy. Even though I had used v-snap to construct the lines, it wasn't happy until I PUT them on the workplane, then had to use "Meet 2 Lines tool to fix them again.
Even with all that, it disappears every time -- just as soon as I make one mouse click after clicking Finish.
To first replyer: I am using v15.2