I just loaded 1.0.0 RC 4 and tried the same thing. It did not crash, but slightly incorrect behavior:
When I open FreeMind (from apps), it opens .mms that were open on the last close. So if instead I open an .mm by using "Open With" (apparently my defaults aren't set for proper double click operation) I get two versions of the .mm in the window, the second identified as ...(2).

Not sure if this could be related to your crash, but thought I'd note it.

Mac OSX 10.6.8
MacBook Pro, 2.2GHz, Core 2 Duo, 3GB DDR2
Java 1.6.0_51