Again, could not reproduce this. Had created a new task and per drag&drop added as an attachment a memo. I don't exactly remember when it crashed: when I tried to open the attachment or when I saved the task. Here is the stacktrace:

(evolution:5759): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `ETasks'

Program received signal SIGSEGV, Segmentation fault.
0xb5e6fdc6 in model_row_changed_cb (etm=0x8e6a118, row=45, data=0x8f51de0) at ../../../../evolution/calendar/gui/e-tasks.c:350

Num     Type           Disp Enb Address    What
1       breakpoint     keep y   0xb5dc7125 in ewv_pass_gdkevent_to_etext at ../../../../evolution/calendar/gui/e-week-view.c:2146
	stop only if (event->spans_index + week_view->editing_span_num >= week_view->spans->len)
(gdb) [Thread 0xb1003b90 (LWP 8284) exited]

Num     Type           Disp Enb Address    What
1       breakpoint     keep y   0xb5dc7125 in ewv_pass_gdkevent_to_etext at ../../../../evolution/calendar/gui/e-week-view.c:2146
	stop only if (event->spans_index + week_view->editing_span_num >= week_view->spans->len)
(gdb) bt

+ Trace 219521

$1 = (ETasksPrivate *) 0x0
(gdb)