Here is the solution
First find $this->WP_Widget( from your directory. For this you can use Find in Files in windows. Find $this->WP_Widget( , select your directory and then click on Find All. Here is the image
After that select that file/files where $this->WP_Widget( found and replace with parent::__construct(. I think your error will be solved.
Enjoy It.
Thank you.
