I just retrieved libedssharp sources and rebuilt (VS2013). Using the build result the Object Dictionary Editor is v0.5.

1. Load the DS301_profile.xml into editor
2. Select Object Dictionary 0x1016 Consumer heartbeat time. The max sub-index starts at 4 and there are 4 subindexs defined.
3. Right click sub-index 0 and select "Add sub item". The max-subindex increments by 1 and the new items appears in the list. (all good!)
4. Continue adding subitems until the max sub-index is at 10.
5. Add one more sub-item : The max sub-item index changes from 10 to 17 even though only one additional sub-item was added to the subindex list. Perhaps the 10 displayed was interpreted somewhere as a hex value (16 + 1) = 17?
6. If you keep on adding sub-items the max sub-item index will increment by larger amounts eventually exceeding the range of UNSIGNED8 and causing the editor to throw and exception.

The work around is to manually reset the max sub-item index value (Right click on sub-index 0 and select the option to change the max sub-index). I've fooled around with this a bit and I'm pretty sure one part of the editor code interprets the max sub-index as a hex value but other parts assume its decimal.

jl