Fix #284: AbortException crashes when called more than once

This change fixes an issue in PowerShellContext.AbortException which
causes it to throw a NullReferenceException when called after the session
has already been aborted.  This issue manifests sometimes when the
debug adapter finishes execution normally or when it terminates due to a
script parsing error.  It was initially reported in the following two
issues for the PowerShell VS Code extension:

PowerShell/vscode-powershell#243
PowerShell/vscode-powershell#273

The fix is to check whether the PowerShellContext.SessionState is Aborted
or Disposed before proceeding with the abort.

Author: daviwil

src/PowerShellEditorServices.Protocol/MessageProtocol/MessageDispatcher.cs

@@ -331,6 +331,12 @@ private void OnListenTaskCompleted(Task listenTask)
         {
             if (listenTask.IsFaulted)
             {
+                Logger.Write(
+                    LogLevel.Error,
+                    string.Format(
+                        "MessageDispatcher loop terminated due to unhandled exception:\r\n\r\n{0}",
+                        listenTask.Exception.ToString()));
+
                 this.OnUnhandledException(listenTask.Exception);
             }
             else if (listenTask.IsCompleted || listenTask.IsCanceled)

src/PowerShellEditorServices.Protocol/Server/DebugAdapter.cs

@@ -201,15 +201,20 @@ protected Task HandleDisconnectRequest(
                     if (e.NewSessionState == PowerShellContextState.Ready)
                     {
                         await requestContext.SendResult(null);
-                        editorSession.PowerShellContext.SessionStateChanged -= handler;
+                        this.editorSession.PowerShellContext.SessionStateChanged -= handler;
 
                         // Stop the server
                         await this.Stop();
                     }
                 };
 
-            editorSession.PowerShellContext.SessionStateChanged += handler;
-            editorSession.PowerShellContext.AbortExecution();
+            // In some rare cases, the EditorSession will already be disposed
+            // so we shouldn't try to abort because PowerShellContext will be null
+            if (this.editorSession != null && this.editorSession.PowerShellContext != null)
+            {
+                this.editorSession.PowerShellContext.SessionStateChanged += handler;
+                this.editorSession.PowerShellContext.AbortExecution();
+            }
 
             return Task.FromResult(true);
         }

src/PowerShellEditorServices/Session/PowerShellContext.cs

@@ -571,7 +571,8 @@ public async Task LoadHostProfiles()
         /// </summary>
         public void AbortExecution()
         {
-            if (this.SessionState != PowerShellContextState.Aborting)
+            if (this.SessionState != PowerShellContextState.Aborting &&
+                this.SessionState != PowerShellContextState.Disposed)
             {
                 Logger.Write(LogLevel.Verbose, "Execution abort requested...");
 
@@ -587,7 +588,8 @@ public void AbortExecution()
             {
                 Logger.Write(
                     LogLevel.Verbose,
-                    "Execution abort requested while already aborting");
+                    string.Format(
+                        $"Execution abort requested when already aborted (SessionState = {this.SessionState})"));
             }
         }