FIX: record dialog stack and EndModal on system session end

Change-Id: Ie0a7de7cf467f5b9e88881d3894dbb7a0790d500

src/slic3r/GUI/GUI_Utils.cpp

@@ -469,5 +469,7 @@ bool generate_image(const std::string &filename, wxImage &image, wxSize img_size
     return true;
 }
 
+std::vector<wxDialog*> dialogStack;
+
 }
 }