I was having the same problem with my Tree View as well but I managed to make something work and hopefully this will help you :)
Tree Item Example
For this example, I have a tree item that needs to be checked in order for my program to proceed to the next stage otherwise it will throw an error "No Item has been selected".
First I find the actual element using FindElement and clicking on it to highlight it:
tcrForm.FindElement(By.Name("JG TF02 - 1 Defects Own Cycle")).Click();
Then by trial and error I moved the mouse coordinates manually using the Actions function. By moving and performing a click at the same time I managed to click the ToggleBox:
Actions action = new Actions(driver); // Try and click the checkbox
// From the centre of the element, move left, then click
action.MoveByOffset(-117, 0).Click().Perform();
Debug.WriteLine("Click has been performed");
I'm sure there is someone out there who has a better solution for this but as far as I know, this works fine. Please message me if it doesn't work for you.