I assume that you made a copy of the Controller widget source code and tried to make a custom Controller widget.
However, the LayoutItemToolbar is an internal component which is not intended for custom widgets. The LayoutItemToolbar displays as a toolbar of the selected widget in the builder. It provides some widget tools, such as "Quick Style"(Button, Divider) and "Edit"(Text). You can remove the code related to LayoutItemToolbar in the custom Controller. This won't affect other parts of the Controller's logic.
Please let me know if you run into any furthur issues. Thanks!
Shen Zhang