fixup! PostClass.IsRetweetを追加, 公式RTでないツイートのRetweetedIdを参照すると例外を返す

Author: upsilon

OpenTween/Tween.cs

@@ -2271,7 +2271,7 @@ private void ContextMenuOperate_Opening(object sender, CancelEventArgs e)
                 this.AuthorContextMenuItem.Visible = true;
                 this.AuthorContextMenuItem.Text = $"@{post!.ScreenName}";
                 this.RetweetedByContextMenuItem.Visible = post.IsRetweet;
-                this.RetweetedByContextMenuItem.Text = $"@{post.RetweetedBy}";
+                this.RetweetedByContextMenuItem.Text = post.IsRetweet ? $"@{post.RetweetedBy}" : "";
             }
             var tab = this.CurrentTab;
             if (tab.TabType == MyCommon.TabUsageType.DirectMessage || !this.ExistCurrentPost || post == null || post.IsDm)
@@ -8906,7 +8906,7 @@ private void MenuItemOperate_DropDownOpening(object sender, EventArgs e)
                 this.AuthorMenuItem.Visible = true;
                 this.AuthorMenuItem.Text = $"@{post!.ScreenName}";
                 this.RetweetedByMenuItem.Visible = post.IsRetweet;
-                this.RetweetedByMenuItem.Text = $"@{post.RetweetedBy}";
+                this.RetweetedByMenuItem.Text = post.IsRetweet ? $"@{post.RetweetedBy}" : "";
             }
 
             if (tab.TabType == MyCommon.TabUsageType.DirectMessage || !this.ExistCurrentPost || post == null || post.IsDm)