Bug Unity "NullReferenceException: Object reference not set to an instance of an object"

Hi guys, I'm coming to you because I have a problem in unity. I explain, I created a script to generate several entries for a Dropdown. When I launch the application, the script will create the multiple entries of my Dropdown, but I receive the following error message:

"NullReferenceException: Object reference not set to an instance of an object initTransaction.initSelectTransaction (Int32 nbOption) (at Assets / Scenes / script / initTransaction.cs: 25) initTransaction.Start () (at Assets / Scenes / script / initTransaction.cs: 10) "

Do you have any idea where it can come from? Thank you guys (I'll put my script below)

Edit : Problem resolve