Open the NeoLemmix.exe. At the "Select Level Pack" dialog, type any random letters in the File Name section that is not a LVL or NXP file (e.g. "imalittleteapot", "askjfalfjka" etc.).
The NeoLemmix player opens, you get a completely blank level that has 0 lemmings, but you have to save 20 of them, with release rate 50 and 5 minutes.
What should happen instead is that you should get an error dialog saying something like "a LVL file or NXP with that name does not exist".
It's basically reverting to the "default" values (no idea why there are even defaults at all; may relate to code in vanilla Lemmix that was shared between the player and the editor). But indeed, it shouldn't let you do that. I'll look into it next time I'm working on the player (doing a bit of Editor stuff at the moment), I expect it'll be a pretty simple fix.
Fixed for next update.