Strip Mask Modifier problem with custom transistions

I was following this tutorial -

but ended up having same problem - like this -

Please help.

Make sure that in scene properties you have background set to sky not transparent. Also did you trim the length of the mask strip on the timeline or is it the entire length of the source video?

The background is set to sky.
How to trim the length of strip Mask?

Please try the tutorial and let me know if it works.

Can anyone help me out with this please…?