PLT-4096 fix error/panic on replying to integrations. (#4968)

api/post.go

@@ -624,13 +624,14 @@ func sendNotifications(c *Context, post *model.Post, team *model.Team, channel *
 				list := result.Data.(*model.PostList)
 
 				for _, threadPost := range list.Posts {
-					profile := profileMap[threadPost.UserId]
+					if profile, ok := profileMap[threadPost.UserId]; ok {
 						if profile.NotifyProps["comments"] == "any" || (profile.NotifyProps["comments"] == "root" && threadPost.Id == list.Order[0]) {
 							mentionedUserIds[threadPost.UserId] = true
 						}
 					}
 				}
 			}
+		}
 
 		// prevent the user from mentioning themselves
 		if post.Props["from_webhook"] != "true" {