fix overload ambiguity when compiler doesn't know NULL is a pointer

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@53586 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/unix/taskbarx11.cpp

@@ -264,7 +264,8 @@ wxTaskBarIcon::~wxTaskBarIcon()
 {
     if (m_iconWnd)
     {
-        m_iconWnd->Disconnect(wxEVT_DESTROY, NULL, NULL, this);
+        m_iconWnd->Disconnect(
+            wxEVT_DESTROY, wxObjectEventFunction(NULL), NULL, this);
         RemoveIcon();
     }
 }