Avoid custom UI error

The "TabMail" tab exists only in the ribbon whose id is "Microsoft.Outlook.Explorer". So if we open a ribbon
whose id is not "Microsoft.Outlook.Explorer", we may get a custom UI error like "TabMail" doesn't exist.
The error is displayed when Outlook's setting "Options" -> "Advanced" -> "Show add-in user interface errors"
is enabled.

This patch avoid that error by creating custom UI only when ribbonID is "Microsoft.Outlook.Explorer".

Ribbon.cs

@@ -43,8 +43,12 @@ public string Ribbon_LoadLabel(Office.IRibbonControl Control)
         #region IRibbonExtensibility のメンバー
 
         public string GetCustomUI(string ribbonID)
-        {
-            return GetResourceText("FlexConfirmMail.Ribbon.xml");
+        {
+            if (ribbonID == "Microsoft.Outlook.Explorer")
+            {
+                return GetResourceText("FlexConfirmMail.Ribbon.xml");
+            }
+            return "";
         }
 
         #endregion