Fix an About dialog–related crash.

When the dialog is closed by the user clicking the X, rather than a
button in the dialog, the dialog doesn't exist anymore, so we can't
simply hide and later re-present it.

CHANGELOG

@@ -1,5 +1,10 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [Zenmap] Fixed a crash that could be caused by opening the About
+  dialog, using the window manager to close it, and opening it again.
+  This was reported by Yashartha Chaturvedi and Jordan Schroeder.
+  [David Fifield]
+
 o [Ncat] Made test-addrset.sh exit with nonzero status if any tests
   fail. This in turn causes "make check" to fail if any tests fail.
   [Andreas Stieger]

zenmap/zenmapGUI/MainWindow.py

@@ -665,10 +665,16 @@ This scan has not been run yet. Start the scan with the "Scan" button first.'))
             self._save_all(self.scan_interface, dir_chooser.get_filename())
         dir_chooser.destroy()
 
+    def _about_cb_response(self, dialog, response_id):
+        if response_id == gtk.RESPONSE_DELETE_EVENT:
+            self._about_dialog = None
+        else:
+            self._about_dialog.hide()
+
     def _show_about_cb(self, widget):
         if self._about_dialog is None:
             self._about_dialog = About()
-            self._about_dialog.connect("response", lambda dialog, response: dialog.hide())
+            self._about_dialog.connect("response", self._about_cb_response)
         self._about_dialog.present()
 
     def _save_all(self, scan_interface, directory):