Add XML comment to public enum type.

Handle possibilty of there not being a InvocationInfo property on a PSObject.

Author: rkeithhill

src/PowerShellEditorServices/Debugging/StackFrameDetails.cs

@@ -8,6 +8,9 @@
 
 namespace Microsoft.PowerShell.EditorServices
 {
+    /// <summary>
+    /// An optional hint for how to present a stack frame in the UI. 
+    /// </summary>
     public enum StackFramePresentationHint
     {
         /// <summary>
@@ -117,7 +120,7 @@ static internal StackFrameDetails Create(
             string moduleId = string.Empty;
             var presentationHint = StackFramePresentationHint.Normal;
 
-            var invocationInfo = callStackFrameObject.Properties["InvocationInfo"].Value as InvocationInfo;
+            var invocationInfo = callStackFrameObject.Properties["InvocationInfo"]?.Value as InvocationInfo;
             string scriptPath = (callStackFrameObject.Properties["ScriptName"].Value as string) ?? NoFileScriptPath;
             int startLineNumber = (int)(callStackFrameObject.Properties["ScriptLineNumber"].Value ?? 0);