Hello,

Perhaps you can try mocking the first-level property, TruckNameTextBox, and assign it a concrete value of the TextBox object instance.

It would look something like:
_mockDialog.Setup(d => d.TruckNameTextBox).Returns(new TextBox { Text = string.Empty", ...});

You tried to mock Text property within TruckNameTextBox, which initially wasn't mocked, causing and exception