Plugins that reference Post via ForeignKey all erroring when trying to add/edit them

I made 3 plugins a few months back, that reference the Post model of this app using the ForeignKey field. I’m not sure what changed, as no changes were made to the code base, but for some reason all of the plugins that have these fields in them are now returning this error:

__str__ returned non-string (type NoneType)

Error during template rendering

In template /home/****/.env/venv/lib/python3.5/site-packages/django/contrib/admin/templates/admin/includes/fieldset.html, error at line 19

__str__ returned non-string (type NoneType)
9	            {% for field in line %}
10	                <div{% if not line.fields|length_is:'1' %} class="field-box{% if field.field.name %} field-{{ field.field.name }}{% endif %}{% if not field.is_readonly and field.errors %} errors{% endif %}{% if field.field.is_hidden %} hidden{% endif %}"{% elif field.is_checkbox %} class="checkbox-row"{% endif %}>
11	                    {% if not line.fields|length_is:'1' and not field.is_readonly %}{{ field.errors }}{% endif %}
12	                    {% if field.is_checkbox %}
13	                        {{ field.field }}{{ field.label_tag }}
14	                    {% else %}
15	                        {{ field.label_tag }}
16	                        {% if field.is_readonly %}
17	                            <p>{{ field.contents }}</p>
18	                        {% else %}
19	
                                  
      {{ field.field }}
      

      
20	                        {% endif %}
21	                    {% endif %}
22	                    {% if field.field.help_text %}
23	                        <p class="help">{{ field.field.help_text|safe }}</p>
24	                    {% endif %}
25	                </div>
26	            {% endfor %}
27	        </div>
28	    {% endfor %}
29	</fieldset>

The model here is very simple:

class FailingModel(CMSPlugin):
    post = models.ForeignKey(Post, related_name="failing_model", on_delete=models.CASCADE)

Any help would be greatly appreciated.

[gmenih]

@yakky Used a backed up database, since I wasn’t able to create a post without a title again, and it works now - shows the title of a different translation for the post that doesn’t exist in current translation.

Thanks for solving this!

[yakky]

@gmenih341 we need a smarter __str__ I guess. Let me check and I’ll be back to you later today