fix: preview: set some dummy title to popup window

src/popup.tab-preview/tab-preview.html

@@ -4,6 +4,7 @@
   <meta charset="utf-8">
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>---</title>
   <style>
     @keyframes appearing {
       0% { opacity: 0; }