Render Error when updating a QQuickItem

I attached the example.

I create a QQuickItem according to the documentation Scene Graph - Custom Geometry. It draws lots of lines to show that there are lots of point pairs. It's called poinpairs.h/cpp in the attachment.

The error occurs  when doing the following actions:

1. Run the application
2. Click 'Button2' to load an image
3. Press and move the mouse in order to 'draw' the line
4. Try Step 3 many times and you will see there aren't any lines (In fact they are just invisible now).
5. Click 'Button2' and the image will load again
6. The orange lines are visible now.
7. Try Step 3 to draw more lines and everything is fine.

In fact you don't even need to click Button2 (to load the image again) in Step 5. If there is a button doing nothing, clicking it will also works. The effect is like this gif.

You may find there is an empty whole in the image after Step 2. It's another bug in this project. It has been reported as QTBUG-70046. These two bugs are independent with each other.

By the way, I create some other QQuickItem. They won't cause this problem. But I'm sure I create this PointPairs carefully according to the documentation and you can check it.