FIX: Check if role selection is valid

frontend/src/views/netdb/Tokens.vue

@@ -748,13 +748,13 @@ export default {
       this.updateCanAddRole()
     },
     updateCanAddRole() {
-      if (!this.new_system || !this.new_role_name) {
+      const new_role = this.new_system + '.' + this.new_role_name
+      if (!this.new_system || !this.new_role_name || !this.assignable_roles_by_fq_name[new_role]) {
         this.can_add_role = false
         this.cant_add_role_reason = 'System & Rolle auswählen'
         return
       }
       let valid = true
-      const new_role = this.new_system + '.' + this.new_role_name
       this.new_account.roles.forEach(role => {
         if (role.fq_name === new_role) {
           valid = false