Need message to assign the defect on view issue details screen

[Garden16]

I am currently using a sub task (defect subtask) to manage defects.  Upon creating the create subtask link to create Sub task , a new a defect is created , I would like to assign the defect  to the developer using the Assign defect  button on the View issue detail screen.

I would like a  a warning message to be displayed  indicating the defect  needs to assigned on the  Issue view details page .

How Do I achieve this using script runner . Alternatively I would like to stop the open defect transition to be opened unless the defect  is assigned. I would like to display an error message if the developer clicks  on Open defect transition . 

I want to make sure that the defect  is not opened unless the defect  is assigned .Do not want it to be Un assigned .

Please suggest script runner solution for these issue .