ウィンドウが先に閉じられてしまっていた場合への対処

git-svn-id: http://www.cozmixng.org/repos/piro/treestyletab/trunk@3194 599a83e7-65a4-db11-8015-0010dcdd6dc2

content/treestyletab/treestyletabbrowser.js

@@ -2294,6 +2294,7 @@ TreeStyleTabBrowser.prototype = {
 	closeOwner : function(aTabOwner) 
 	{
 		var w = aTabOwner.ownerDocument.defaultView;
+		if (!w) return;
 		if ('SplitBrowser' in w &&
 			'getSubBrowserFromChild' in w.SplitBrowser) {
 			var subbrowser = w.SplitBrowser.getSubBrowserFromChild(aTabOwner);