I tried to download the Quest, and this error popped up when I opened it.

   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at TextAdventures.Quest.EditorControls.WFEditorTree.SetMenuEnabled(String key, Boolean enabled)
   at TextAdventures.Quest.Editor.UpdateClipboardButtons()
   at TextAdventures.Quest.Editor.ShowEditor(String key)
   at TextAdventures.Quest.Editor.ctlTree_SelectionChanged(String key)
   at TextAdventures.Quest.EditorControls.WFEditorTree.ChangeSelection(String key)
   at TextAdventures.Quest.EditorControls.WFEditorTree.SelectCurrentTreeViewItem()
   at TextAdventures.Quest.EditorControls.WFEditorTree.ctlTreeView_AfterSelect(Object sender, TreeViewEventArgs e)
   at System.Windows.Forms.TreeView.OnAfterSelect(TreeViewEventArgs e)
   at System.Windows.Forms.TreeView.TvnSelected(NMTREEVIEW* nmtv)
   at System.Windows.Forms.TreeView.WmNotify(Message& m)
   at System.Windows.Forms.TreeView.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)"

Did I do something wrong?

I am having the exact same issue and idk what happened. I had just updated it and it pops up and it's pretty annoying. I hope somebody know how to fix this because I cannot create anything if that continues happening.

I just figured out what you have to do. You need to go to your control panel and uninstall Quest. Don't worry, all your files will be safe but even still I recommend to back your files up before continuing. Then you will have to uninstall it and proceed to run the Quest 5.8 installer again. Say yes to everything and then run it again, it should fix the problem (it did for me).

Thank you! I'll try it out.