not able to select multiple options from the drop down

I feel like you're doing something funny with your local variable.  How is it initially declared / what is its initial value?  Why the a!defaultValue() wrapper?  And why, for a variable that inherently must be a list type, are you making it default to a single-type "null()"?  It seems like you should be defaulting it to an empty set ({}), if anything.

I feel like you're doing something funny with your local variable.  How is it initially declared / what is its initial value?  Why the a!defaultValue() wrapper?  And why, for a variable that inherently must be a list type, are you making it default to a single-type "null()"?  It seems like you should be defaulting it to an empty set ({}), if anything.