Fix SEGV in ErrorScreenHandler::HandleHideCaptivePortal.

BUG=475936

Review URL: https://codereview.chromium.org/1086563003

Cr-Commit-Position: refs/heads/master@{#325213}

chrome/browser/ui/webui/chromeos/login/error_screen_handler.cc

@@ -76,7 +76,8 @@ bool ErrorScreenHandler::GetScreenName(OobeUI::Screen screen,
 }
 
 void ErrorScreenHandler::HandleHideCaptivePortal() {
-  model_->HideCaptivePortal();
+  if (model_)
+    model_->HideCaptivePortal();
 }
 
 void ErrorScreenHandler::RegisterMessages() {