gluon-web: fix access to undefined in checkvalue()

Fixes: cfe1bba8 "gluon-web: fix radio button view of ListValues"

package/gluon-web/javascript/gluon-web.js

@@ -159,7 +159,8 @@
 			return (value == ref);
 		} else {
 			t = document.getElementById(target + '.' + ref);
-			return (t.type == "radio" && t.checked);
+			if (t)
+				return (t.type == "radio" && t.checked);
 		}
 
 		return false;