Fix #357: Watch evaluation causes debugger hang

This issue is caused when VS Cod sends watch expression evaluation
requests after the debugger has been resumed.  This issue has been fixed
in the most recent builds of VS Code but we must have a guard for this on
our side for those who are still using VS Code Stable.

Author: daviwil

src/PowerShellEditorServices.Protocol/Server/DebugAdapter.cs

@@ -764,11 +764,17 @@ protected async Task HandleEvaluateRequest(
             }
             else
             {
-                VariableDetails result =
-                await editorSession.DebugService.EvaluateExpression(
-                    evaluateParams.Expression,
-                    evaluateParams.FrameId,
-                    isFromRepl);
+                VariableDetails result = null;
+
+                // VS Code might send this request after the debugger
+                // has been resumed, return an empty result in this case.
+                if (editorSession.PowerShellContext.IsDebuggerStopped)
+                {
+                    await editorSession.DebugService.EvaluateExpression(
+                        evaluateParams.Expression,
+                        evaluateParams.FrameId,
+                        isFromRepl);
+                }
 
                 if (result != null)
                 {