title: How do I represent a void pointer in a PyObjC selector?
body: <p>I'm wanting to use an NSOpenPanel for an application I'm designing.  Here's what I have so far:</p>
  
  <pre><code>@objc.IBAction
  def ShowOpenPanel_(self, sender):
      self.panel = NSOpenPanel.openPanel()
      self.panel.setCanChooseFiles_(False)
      self.panel.setCanChooseDirectories_(True)
      NSLog(u'Starting OpenPanel')
      self.panel.beginForDirectory_file_types_modelessDelegate_didEndSelector_contextInfo_(
          self.defaults.objectForKey_(u'projpath'), 
          objc.nil, 
          objc.nil, 
          self, 
          objc.selector(self.OpenPanelDidEnd_returnCode_contextInfo_, 
              signature='v:@ii'),
          objc.nil)
      NSLog(u'OpenPanel was started.')
  
  def OpenPanelDidEnd_returnCode_contextInfo_(self, panel, returnCode, context):
      NSLog('Panel ended.')
      if (returnCode == NSOKButton):
          NSLog(u'User selected OK')
          path = self.panel.filenames()[0]
          self.defaults.setObject_forKey_(path, u'projpath')
      del self.panel
  </code></pre>
  
  <p>The main two lines I'm concerned about are:</p>
  
  <pre><code>        objc.selector(self.OpenPanelDidEnd_returnCode_contextInfo_, 
              signature='v:@ii'),
          objc.nil) #this is the argument that gets passed as the void pointer
  </code></pre>
  
  <p>The third argument is supposed to be a void pointer.  Since I don't intend to use that data, I'd rather just leave it empty.  I've tried making the signature <code>'v:@iv'</code> and tried using <code>objc.NULL</code> and python's <code>None</code>, and just about every combination of all these things.  What is the best way to handle this?</p>