don't use annoying and unneeded in C++ casts of NULL to "T *" in all other files neither

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58227 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/motif/msgdlg.cpp

@@ -156,7 +156,7 @@ int wxMessageDialog::ShowModal()
 
     if (m_backgroundColour.Ok())
     {
-        wxComputeColours (dpy, & m_backgroundColour, (wxColour*) NULL);
+        wxComputeColours (dpy, & m_backgroundColour, NULL);
 
         XtSetArg(args[ac], XmNbackground, g_itemColors[wxBACK_INDEX].pixel); ac++;
         XtSetArg(args[ac], XmNtopShadowColor, g_itemColors[wxTOPS_INDEX].pixel); ac++;