"Index was out of range" error when adding structure in GUI tool

[mattl91]

An "Index was out of range" error occurs if you select "Add Structures" and then attempt to use down arrow followed by ENTER to select a structure in the Structures tab of the GUI tool.

To Reproduce
Steps to reproduce the behavior:

1. Create a solution by using the harmonycore template (e.g., dotnet new harmonycore -o MyApi)
2. Open the GUI tool for the new solution.
3. Go to the Structures tab and add the Customers structure.
4. After the Add Structures screen is closed and your back In the Structures tab, press CTRL+A to open the Add Structures window and then press the down arrow to highlight ITEMS, and then press ENTER.
5. Notice the "Failed to set text" window with it's message: "Index was out of range. Must be non-negative and less than the size of the collection. (Parameter "index")" .
6. Click OK in the error message window.
7. Notice that the Index structure was not added.

Expected behavior
No error should occur and the INDEX structure should be added.

CLI tool version

• 6.0.158