#3 crochetter crashes when adding slip stitch to chain in end

[Rikard Edgren]

Client: Windows 7 SP1, Windows Update as of 2011-10-25.

1. Launch crochetter.exe 0.0.1
2. Click on chain in toolbar
3. Add a chain in main window
4. Click on slip stitch in toolbar
5. Click on chain in main window

Result: Crochetter crashes.

Expected: Not sure if this is allowed, but maybe an informative error message would be appropriate.

Note: This crash also appears when adding slip stitch to the end of bigger chains.
It can also appear when multi-clicking slip stitches on chains.

Found and reported by YhPV11 - vocational study for software testers in Karlstad.