Fix a null dereference when an invalid cast exception has no inner exception (#729)

Author: rjmholt

src/PowerShellEditorServices/Console/InputPromptHandler.cs

@@ -275,8 +275,7 @@ private async Task<Dictionary<string, object>> StartPromptLoop(
                     }
                     catch (PSInvalidCastException e)
                     {
-                        this.ShowErrorMessage(e.InnerException);
-
+                        this.ShowErrorMessage(e.InnerException ?? e);
                         continue;
                     }
                 }