Fix ISE while deleting a DS created from a DSR

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 ### Fixed
 - Update traffic_portal dependencies to mitigate `npm audit` issues.
 - Fixed a cdn-in-a-box build issue when using `RHEL_VERSION=7`
+- [#6549](https://github.com/apache/trafficcontrol/issues/6549) Fixed internal server error while deleting a delivery service created from a DSR (Traafic Ops).
 
 ### Removed
 - Remove traffic_portal dependencies to mitigate `npm audit` issues, specifically `grunt-concurrent`, `grunt-contrib-concat`, `grunt-contrib-cssmin`, `grunt-contrib-jsmin`, `grunt-contrib-uglify`, `grunt-contrib-htmlmin`, `grunt-newer`, and `grunt-wiredep`

traffic_ops/traffic_ops_golang/deliveryservice/request/assign.go

@@ -223,10 +223,11 @@ func PutAssignment(w http.ResponseWriter, r *http.Request) {
 	}
 	var resp interface{}
 	if inf.Version.Major >= 4 {
-		if dsr.Requested.LongDesc1 != nil || dsr.Requested.LongDesc2 != nil {
+		if dsr.Requested != nil && (dsr.Requested.LongDesc1 != nil || dsr.Requested.LongDesc2 != nil) {
 			api.HandleErr(w, r, tx, http.StatusBadRequest, errors.New("the longDesc1 and longDesc2 fields are no longer supported in API 4.0 onwards"), nil)
 			return
 		}
+
 		if dsr.Original != nil {
 			*dsr.Original = dsr.Original.RemoveLD1AndLD2()
 		}