The "value" property cannot be null. Refer to the documentation for the allowed values:
value | String | Specifies the value to include in the roles claim in ID tokens and access tokens authenticating an assigned user or service principal. Must not exceed 120 characters in length. Allowed characters are : ! # $ % & ' ( ) * + , - . / : ; < = > ? @ [ ] ^ + _ ` { | } ~ , and characters in the ranges 0-9 , A-Z and a-z . Any other character, including the space character, aren't allowed. May not begin with . . |