Can only concatenate str (not "NoneType") to str in Django, what's wrong?

Question

Django

Views code

from django.shortcuts import render
from django.http import HttpResponse

from .models import Board
# Create your views here.
def index(request):
    s = 'СМР Блог\r\n\r\n\r\n'
    for b in Board.objects.order_by('-published'):
        s += b.title + '\r\n' + b.content + '\r\n'
        return HttpResponse(s, content_type = 'text/plain; charset = utf-8')

Urls code

from django.urls import path

from .views import index

urlpatterns = [
    path('board/', index)#, name = 'index')
]

Code Models

from django.db import models

# Create your models here.
class Board(models.Model):
    title = models.CharField(max_length = 120)
    content = models.TextField(max_length = 6000)
    published = models.DateTimeField(auto_now_add = True, db_index = True)

Mistake

TypeError: can only concatenate str (not "NoneType") to str

What is the problem? Be grateful for the answers

Answer 1

Does the database have boards whose title or content is null?

Answer 2

Traceback probably told everything. Most likely there is a problem: In b.title or in b.content instead of the string None. Use f-strings:
s += b.title + '\r\n' + b.content + '\r\n'
s += f'{b.title}\r\n{b.content}\r\n'