[crash][TransWindow::TerminateTransTask] crash on terminate and already finished job

[lidaobing]

How to reproduce

1. send a file to yourself
2. wait for file sending finished.
3. open the Trans Window
4. right click on the file and select "Terminate Task"

what expected:

1. "Terminate Task" should be gray(unclickable)
2. nothing happend when it has been clicked.

what happend:

1. crash as follows

(lldb) bt all
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x18)
  * frame #0: 0x0000000100065be5 iptux`iptux::TerminateTransTask(model=0x000000010202a950) at TransWindow.cpp:321
    frame #1: 0x0000000100b4df8a libgobject-2.0.0.dylib`g_closure_invoke + 198
    frame #2: 0x0000000100b6145c libgobject-2.0.0.dylib`signal_emit_unlocked_R + 2347
    frame #3: 0x0000000100b61df2 libgobject-2.0.0.dylib`g_signal_emit_valist + 1816
    frame #4: 0x0000000100b624dc libgobject-2.0.0.dylib`g_signal_emit + 120
    frame #5: 0x0000000100411496 libgtk-3.0.dylib`gtk_widget_activate + 71
    frame #6: 0x00000001002fda8f libgtk-3.0.dylib`gtk_menu_shell_activate_item + 252
    frame #7: 0x00000001002feb79 libgtk-3.0.dylib`gtk_menu_shell_button_release + 499
    frame #8: 0x00000001002e387f libgtk-3.0.dylib`_gtk_marshal_BOOLEAN__BOXEDv + 189
    frame #9: 0x0000000100b4e15b libgobject-2.0.0.dylib`_g_closure_invoke_va + 210
    frame #10: 0x0000000100b61b44 libgobject-2.0.0.dylib`g_signal_emit_valist + 1130
    frame #11: 0x0000000100b624dc libgobject-2.0.0.dylib`g_signal_emit + 120
    frame #12: 0x0000000100410b69 libgtk-3.0.dylib`gtk_widget_event_internal + 256
    frame #13: 0x00000001002e249b libgtk-3.0.dylib`propagate_event + 364
    frame #14: 0x00000001002e1841 libgtk-3.0.dylib`gtk_main_do_event + 966
    frame #15: 0x00000001007dbc69 libgdk-3.0.dylib`_gdk_event_emit + 49
    frame #16: 0x00000001008007e8 libgdk-3.0.dylib`gdk_event_dispatch + 50
    frame #17: 0x0000000100bb6e53 libglib-2.0.0.dylib`g_main_context_dispatch + 258
    frame #18: 0x0000000100bb7151 libglib-2.0.0.dylib`g_main_context_iterate + 426
    frame #19: 0x0000000100bb71a9 libglib-2.0.0.dylib`g_main_context_iteration + 55
    frame #20: 0x0000000100a39c02 libgio-2.0.0.dylib`g_application_run + 468
    frame #21: 0x000000010000831f iptux`iptux::Application::run(this=0x00007ffeefbff5b0, argc=1, argv=0x00007ffeefbff6c0) at Application.cpp:140
    frame #22: 0x0000000100005d01 iptux`main(argc=1, argv=0x00007ffeefbff6c0) at iptux.cpp:147
    frame #23: 0x00007fff78ae9115 libdyld.dylib`start + 1

[lidaobing]

iptux/src/iptux/TransWindow.cpp

Line 321 in 8e470cc

if (trans) trans->TerminateTrans();