When left-clicking clicking on a tree entry, xffm does not check if it's path is NULL, as it does for right-clicking. So clicking on "Drag here to bookmark" or on the empty line below an opened unmounted device directory causes sig11.
Additional information: An example patch follows. It has been tested on an i686-pc-linux-gnu system. The lines mentioned can still be selected, but that's OK, since xffm automatically disables all operations with them.
Patch applied to CVS HEAD and 4_0 branch.