Do not crash when card is inserted and the user list view is hidden

diff --git a/tdm/kfrontend/kgreeter.cpp b/tdm/kfrontend/kgreeter.cpp
index a42ffd0..5da8bfc 100644
--- a/tdm/kfrontend/kgreeter.cpp
+++ b/tdm/kfrontend/kgreeter.cpp
@@ -916,7 +916,9 @@
 		if (!user_active && vt_active) {
 			// Select the correct user
 			verify->setUser(login_name);
-			userView->setEnabled(false);
+			if (userView) {
+				userView->setEnabled(false);
+			}
 			verifySetUser(login_name);
 			verify->lockUserEntry(true);