Fixed a crash on language changing

(DiffPresetDialog should get mainframe as a parent explicitly)

src/slic3r/GUI/MainFrame.cpp

@@ -116,6 +116,7 @@ DPIFrame(NULL, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, wxDEFAULT_FRAME_S
     m_printhost_queue_dlg(new PrintHostQueueDialog(this))
     , m_recent_projects(9)
     , m_settings_dialog(this)
+    , diff_dialog(this)
 {
     // Fonts were created by the DPIFrame constructor for the monitor, on which the window opened.
     wxGetApp().update_fonts(this);