I’ve tried to reproduce the issue. As you describe, editing a preset without TCurve does NOT work as expected. When Editing the preset, I entered 255 in the upper y value field, which made DPL crash instantly. macOS then popped up the window with the bug report.
Update: I can reproduce the bug with DPL 4.1.1 on Mojave and Big Sur. Bug absent in DPL3.
The text of the report is attached herewith. Crash on Tonecurve Edit.zip (30.9 KB)
Note: DPL offers to create a new “empty preset”. When created and left as is, the preset does not do anything (expected) and does not register in history (unexpected). One must first click to edit the preset before it can be applied. See separate post for improvement.