Fix null dereference bug in PopPowerShell()

Author: andyleejordan

src/PowerShellEditorServices/Services/PowerShell/Host/PsesInternalHost.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation.
+// Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
 using System;
@@ -502,9 +502,11 @@ private void PopPowerShell(RunspaceChangeAction runspaceChangeAction = RunspaceC
             PowerShellContextFrame frame = _psFrameStack.Pop();
             try
             {
-                // If we're changing runspace, make sure we move the handlers over
+                // If we're changing runspace, make sure we move the handlers over.
                 RunspaceFrame previousRunspaceFrame = _runspaceStack.Peek();
-                if (previousRunspaceFrame.Runspace != CurrentPowerShell.Runspace)
+                // Because the frame has been popped, we cannot rely on 'CurrentPowerShell.Runspace'
+                // as it may be empty.
+                if (previousRunspaceFrame.Runspace != frame.PowerShell.Runspace)
                 {
                     _runspaceStack.Pop();
                     RunspaceFrame currentRunspaceFrame = _runspaceStack.Peek();