In my attempt to reproduce bug 7, I found another crashing bug that's a bit different, but I have no idea if the cause is the same or just related.
1) Open Sum-It to start fresh with a blank spreadsheet
2) Type 'a', hit Enter to go to the next cell down
3) Type 'b', hit Enter to go to the next cell down
4) Hit Alt+F, type 'b', and hit Enter
4b) The 'b' cell is found, but now Sum-It is greyish everywhere, the same way it looks normally when the window is visible but does not have the focus; clicking any cell (with no modifier keys, just a simple left-click) will add that individual cell to the current selection of cells
5) Whether you click cells or not, there are now two ways to get Sum-It to crash: either try to close the window, or even just make the window lose focus by clicking somewhere outside of it, and then it will crash when you click its greyed yellow tab to regain focus
Interestingly, if you type 'a' in step 4, the behaviour of 4b and 5 does not occur, and Sum-It chugs along with its normal stability.
This is on Haiku nightly hrev46756 with Sum-It as installed through HaikuDepot, which says that it's Sum-It 0.2beta-1.
Attached is a saved crash report.