fix popup behaviour in firefox with touchscreens on desktop

when the popup blocker is active, nw is undefined and nw.focus() throws
an error and the touchend event is not finished, causing
extjs to retry the touchevent and click handler, which tries to open
a popup which gets blocked here, and so on

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

Utils.js

@@ -586,8 +586,10 @@ Ext.define('Proxmox.Utils', { utilities: {
 	    node: nodename
 	});
 	var nw = window.open("?" + url, '_blank', 'toolbar=no,location=no,status=no,menubar=no,resizable=yes,width=800,height=420');
+	if (nw) {
 	    nw.focus();
 	}
+    }
 
 },