Fix deadlock when coming out of suspension on the UI thread in Ready state

On the change to Ready state, the CE dispatches work to the UI thread and waits for the completion. If the change was triggered from the UI thread, this causes a deadlock. Usually, such a change only happens during parsing, which is executed on a background thread. However, when coming out of parser suspension, it can happen as well. To deal with this, the state changed action now also checks that the old state was not Busy.

Author: MDoerner

Rubberduck.Core/Navigation/CodeExplorer/CodeExplorerViewModel.cs

@@ -13,6 +13,7 @@
 using Rubberduck.UI.Command;
 using Rubberduck.VBEditor.SafeComWrappers;
 using System.Windows;
+using System.Windows.Forms;
 using System.Windows.Input;
 using Rubberduck.Parsing.UIContext;
 using Rubberduck.Templates;
@@ -229,10 +230,11 @@ private void HandleStateChanged(object sender, ParserStateEventArgs e)
         {
             Unparsed = false;
 
-            if (e.State == ParserState.Ready)
+            if (e.State == ParserState.Ready && e.OldState != ParserState.Busy)
             {
                 // Finished up resolving references, so we can now update the reference nodes.
                 //We have to wait for the task to guarantee that no new parse starts invalidating all cached components.
+                //CAUTION: This must not be executed from the UI thread!!!
                 _uiDispatcher.StartTask(() =>
                 {
                     var referenceFolders = Projects.SelectMany(node =>