The security validation for this page is invalid. Click Back in your Web browser…

When making a custom provisioner for a site template I got this error:
“The security validation for this page is invalid. Click Back in your Web browser…”

There is an easy way to avoid this.
This is some sample code:

 public override void Provision(SPWebProvisioningProperties props)
        {
            props.Web.ApplyWebTemplate("TESTDATABASE#0");
            SPSecurity.RunWithElevatedPrivileges(delegate()
            {
                using (SPSite siteCollection = new SPSite(props.Web.Site.ID))
                {
                    using (SPWeb web = siteCollection.OpenWeb(props.Web.ID))
                    {
                        web.Title = props.Data;
                        web.Update();
                        //Create sts child sites
                        web.Webs.Add("Childsite1", "Child Site 1", "", 1033, "STS#1", false, false);
                        web.Webs.Add("Childsite2", "Child Site 2", "", 1033, "STS#1", false, false);
                    }
 
                }
            });
        }

This code failed for me. I then added AllowUnsafeUpdates and It worked well. You add it to both SPSite and SPWeb objects.

 public override void Provision(SPWebProvisioningProperties props)
        {
            props.Web.ApplyWebTemplate("TESTDATABASE#0");
            SPSecurity.RunWithElevatedPrivileges(delegate()
            {
                using (SPSite siteCollection = new SPSite(props.Web.Site.ID))
                {
                    siteCollection.AllowUnsafeUpdates = true;                                        
                    using (SPWeb web = siteCollection.OpenWeb(props.Web.ID))
                    {
                        web.AllowUnsafeUpdates = True;
                        web.Title = props.Data;
                        web.Update();
                        //Create sts child sites
                        web.Webs.Add("Childsite1", "Child Site 1", "", 1033, "STS#1", false, false);
                        web.Webs.Add("Childsite2", "Child Site 2", "", 1033, "STS#1", false, false);
                    }
 
                }
            });
        }