Thanks for your report, Tao. I am not able to reproduce this on my PC, but will keep an eye on the problem whenever I am testing the software on different configurations. Eventually I will solve it
I have some more info on the bug.
It looks like it has to do with the routine(s) that draw the equation entry box. So for example if you click say 3 pixels away from the first box, nothing happens, it's probably an error trap on purpose. But farther away if you click in the white area it deletes the first entry box (assume you did nothing with it), and creates a new one. On my machine those two actions are not instantaneous. Now the bug seems to be related to clicking just far enough away from the first entry box that the program wants to spawn a new instance, but right about that point if you're also holding the left button and moving the mouse, it seems to catch a bad double instance and that's where the fatal crash shows up.
Does that help? It's pretty repeatable here, and even after a reboot.