I appreciate your narrowing down the circumstances where this crash occurs.  The problem was cause by removing a variable that was highlighted in the Watch list in the Debug panel.  When the program was stepped again, Simply Fortran would clear the highlights.  The problem occurred because, although the Watch expression had been removed, the reference to it in the list of currently highlighted watch expressions had not been removed.  Therefore, because of a bit of careless code, Simply Fortran attempted to clear the highlight for a Watch expression that didn't exist any longer, and a memory violation occurred.

Build 1809 is currently being uploaded, and it should fix the problem.

- step in the debugger to the print statement
- add "a" to the watch list (select it, right-click, debugging menu)
- switch to the watch list in the Left-hand panel to view "a"
- remove "a" from the list of expressions (right-click, remove)
- switch back to see the local variables.
- click continue (Shift-F7)

I get "fwin.exe has stopped working" and the GUI crashes.

program main
   real :: a
   
   a = 1.0

   ! Add 'a' to watchlist, then delete  it again
   print *, a

end program main