I finally got around to checking this and I figured out how to reproduce this error...
1. Install the Curvature mod and launch Moho
2. Create a shape and make sure Show Bezier Handles is enabled.
3. Save the file and quit Moho.
4. Launch Moho and reopen the file.
At this stage, the Bezier handles should be visible and you should be able to edit the handles normally. Here's how to make things go wrong...
5. Select the modified Curvature tool and uncheck Show Bezier Handles
6. Save the file and quit Moho
7. Launch Moho and reopen the file.
8. The Bezier handles are hidden as expected. Enable Show Bezier handles.
Now it's impossible to edit the Bezier handles. Even though the handles are visible, Moho behaves the way it does when the handles are hidden. Somehow Moho is getting confused about the visibility state of the Bezier handles.
To un-confuse Moho, do this following...
9. Click to the Curvature tool, disable then reenable Show Bezier Handles.
10. Save the file and quit Moho
11. Launch Moho and reopen the file.
The handles are visible as expected and you should be able to edit them as normal again.
I'm guessing that when Show Handles is disabled from the Curvature options, it's sending incomplete information about the current mode.
I kinda suck at scripting but this might be a good practice project for me to dabble with tonight.